The Concept Library: Plain-Language Definitions with Sources

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1214 concepts for people who make software.
AI & Engineering
New
Model Context Protocol (MCP)
An open standard that lets an AI application find and use outside tools and data through one shared set of messages, so each tool needs one server instead of many connectors.
Psychology
New
Active Listening
A way of listening in which you aim to grasp the full meaning of what someone says, including the feeling, and check it by saying it back in your own words.
AI & Engineering
API
An agreed set of rules that lets one program ask another to do something, without knowing how the other works inside.
AI & Engineering
Large Language Model (LLM)
A neural network trained on very large amounts of text to predict what comes next, then adjusted with human feedback so it follows instructions.
AI & Engineering
Latency
The time between sending a request to a system and receiving its response, summed over network travel, waiting and processing.
AI & Engineering
Transformer
A neural network architecture that processes a whole sequence at once, letting every token draw information directly from every other token through attention.
Design
User Interface
A user interface is the part of a machine or program that people use to control it and to see what it is doing.
Design
Friction
The effort, delay or difficulty a person meets while trying to finish a task in a product.
Product
Feedback Loops
A chain of cause and effect in which a system's output returns as an input and changes the next output.
A/B Testing
Product
A/B testing is a controlled experiment that randomly splits traffic between two versions of a product to measure which one changes a target metric.
Product
User Experience Design
User experience design shapes how a person's whole encounter with a product or service works and feels, so they reach their goals with little effort.
Design
UX Principles
UX principles are general guidelines, such as consistency, feedback and user control, that designers use to judge whether a product is easy and clear to use.
AI & Engineering
Deep Learning
Deep learning trains neural networks with many layers to learn useful features directly from raw data such as images, audio and text.
AI & Engineering
Modularity
Modularity is dividing a system into modules that each hide one design decision behind a stable interface, so that decision can change without touching other modules.
AI & Engineering
Foundation Model
A large AI model trained once on broad data so it can be adapted, with prompts or extra training, to many different tasks.
Design
Human-Computer Interaction
Human-computer interaction is the field that studies how people use computer systems and designs those systems so people can use them correctly.
Psychology
Mental Models
A mental model is a person's simplified, private picture of how something works, which they run in their head to predict outcomes and decide what to do.
AI & Engineering
AI Agent
A system in which a language model chooses its own steps, calls tools, reads the results and repeats until a goal is met or a limit stops it.
AI & Engineering
Deployment
Deployment is moving a built version of software onto the machines where it runs, then configuring it, starting it and checking it works.
Design
Hick's Law
The more options a person has, the longer the choice takes, but each doubling of the options adds only a fixed extra delay.
Design
Personalization
A design approach in which the system changes what each person sees, using data about that person, rather than the person choosing settings.
Design
Visibility of System Status
Visibility of system status is Nielsen's first usability heuristic: an interface should show people what it is doing, soon enough to be useful.
Design
Consistency and Standards
Consistency and standards is Nielsen's fourth usability heuristic: one word, symbol or action means one thing, and the product follows platform conventions.
Design
Dark Patterns
Interface designs that lead people into choices they did not intend, such as unwanted purchases, subscriptions or data sharing.
AI & Engineering
Responsive Design
A way of building one website whose layout, images and text adjust to fit any screen width, using fluid grids, flexible images and media queries.
Design
Accessibility
Accessibility is how well people with disabilities can perceive, understand, navigate, and use a product, service, or place.
Design
Call to Action
A call to action is the instruction in a message, page or advertisement that tells the audience the one next step to take now.
Psychology
Confirmation Bias
The habit of choosing, reading and recalling evidence in ways that keep a belief you already hold from being tested fairly.
Business
Correlation and Causation
The observation that two variables move together, and the separate, often false, claim that one of them causes the other.
Design
Dark Mode
A colour scheme that shows light text and icons on a dark background, offered as a setting beside the light version.
Design
Error Prevention
Designing an interface so that mistakes are hard or impossible to make, instead of relying on error messages after the fact.
Design
Information Hierarchy
The ranking of a page's content by how much the reader needs each piece, so the most important point comes first.
Product
Iterative Design
A design method in which a team builds a version, tests it with users, fixes what the tests reveal, and repeats.
Business
Product Market Fit
The match between a product and a market that wants it enough to use it, pay for it and recommend it.
Design
Affordance
An affordance is an action that an object or environment makes possible for a particular person or animal, whether or not they notice it.
AI & Engineering
Algorithmic Bias
A repeatable error in an automated system that gives one group of people worse results than another, usually traced to its data, target or testing.
Product
Design Thinking
A problem-solving method that starts with the people affected: observe them, define the real problem, then prototype and test ideas in loops.
AI & Engineering
Hallucination
When an AI model generates fluent, confident text that is factually wrong or unsupported by its source material, such as a fabricated citation or invented statistic.
Business
Opportunity Cost
Opportunity cost is the value of the best alternative given up when you choose one option, counting only the next-best choice, not every option rejected.
Design
Visual Hierarchy
Visual hierarchy is the arrangement of design elements so the eye meets them in the order of their intended importance, using contrast, size and grouping.
Research
Accuracy
How close a measurement or prediction is to the true value; in classification, the share of predictions that are correct.
Design
Contrast
The measurable difference in luminance, size or weight between an element and its surroundings that makes it readable and noticeable.
Design
Design System
The maintained set of standards, components and code a team uses to design and build a product consistently at scale.
Design
Discoverability
Discoverability is how easily a person can work out which actions are possible, and where and how to perform them, from the design alone.
Psychology
Dopamine Loops
A repeating cycle in which a cue prompts an action, the outcome adjusts how strongly the cue motivates, and dopamine signals support wanting and learning.
Design
Mobile First Design
A method of designing and building a product for the phone-sized screen first, then adding content and features for larger screens.
Psychology
Social Proof
The tendency of people to copy what others do when unsure how to act, used in marketing by showing ratings, counts and popular choices.
Product
Aha Moment
The first time a new user experiences a product's core value, usually measured by one early action that predicts who stays.
Leadership
Diversity, Equity, and Inclusion (DEI)
DEI is the practice of bringing in people of many backgrounds, giving each what they need for equal opportunity, and making sure all can take part.
Design
Microinteractions
A microinteraction is one small moment built around a single task, where a trigger starts something and the product answers with feedback.
Business
Pareto Principle
The Pareto Principle says that in many systems a small share of causes, often about 20 percent, produces most of the results, often about 80 percent.
Leadership
Psychological Safety
A team's shared belief that members can ask, disagree, admit mistakes and suggest rough ideas without being punished or embarrassed.
Design
System and Real World Alignment
System and real world alignment is the usability rule that an interface should use the words, symbols, controls and order its users already know from daily life.
Research
UX Research
UX research is the study of the people who use a product, to learn what they need and do so design decisions rest on evidence.
Design
Whitespace
Whitespace is the empty space between and around elements in a layout, used to group, separate and emphasise content and to make text easier to read.
Design
Aesthetic and Minimalist Design
Aesthetic and minimalist design is Nielsen's eighth usability heuristic: a screen should not show information that is irrelevant or rarely needed.
Design
Cognitive Ease
The feeling that information is easy to process, which people mistake for a sign that it is true, safe or simple.
Psychology
Default Effect
The tendency to keep whichever option is pre-selected instead of choosing another, so the same choice ends up different depending on the default.
Design
Emotional Design
Don Norman's model of how feelings at three levels, visceral, behavioral and reflective, shape how people judge and use a product.
Business
Exploration / Exploitation
The trade-off between using an option already known to work and trying uncertain options that might work better.
AI & Engineering
Guardrails
Checks placed around an AI model that block, change or flag the inputs and outputs that break a set of rules.
Design
Ideation
The stage of a design process in which a person or team generates many possible solutions to a defined problem before judging any of them.
Psychology
Loss Aversion
People feel a loss more strongly than a gain of the same size, so they work to keep what they have more than to get something new.
Design
Recognition rather than Recall
A usability heuristic: show people their options instead of making them remember commands, items and earlier entries.
Business
Return on Investment
Return on investment is the gain from an investment divided by its cost, shown as a percentage, so investments of different sizes can be compared.
AI & Engineering
Throughput
The amount of work a system finishes in a given period of time, such as requests per second, counting only completed work.
Design
User Flow
A diagram of the steps and decisions one person goes through in a product to finish one task.
Product
User stories
A user story is a short note of who wants to do what with a product and why, kept as a reminder to talk and agree when the work is done.
Design
Conceptual Models
A conceptual model is the picture of how a system works, in terms of its objects, actions and rules, that a designer builds into a product for users to learn.
AI & Engineering
Context Window
The total amount of text a language model can consider in a single request, measured in tokens and shared between the input and the response.
Psychology
Decision Fatigue
The claim that choices get worse after many decisions in a row, so people pick the easiest option.
Design
Grid Systems
A structure of columns, rows, gutters and margins that gives every element on a page or screen a fixed place and width.
Design
Reflective Design
Reflective design is designing for what a product means to its owner: the memories, story and self-image it carries.
AI & Engineering
Retrieval-Augmented Generation
A method that answers a question by searching a document collection first, then having a language model write the answer from the passages it found.
Leadership
Second-order effects
The results that follow from the direct results of a decision, usually because other people change what they do in response.
Design
8-Point Grid
The 8-point grid is a spacing rule that sets every height, width, margin and padding in an interface to a multiple of 8.
Design
Form Follows Function
A design principle that says the shape of a building or object should come from what it has to do, so its appearance shows its purpose.
Design
Inclusive Design
Inclusive design is the practice of designing so that people with a wide range of abilities, languages, ages and circumstances can use a product, service or place.
Psychology
Aesthetic-Usability
The tendency of people to judge an attractive interface as easier to use than an unattractive one, even when both work equally well.
AI & Engineering
AI Safety
The field that works to prevent or reduce harm caused by AI systems, from misuse, from accidents and from loss of control.
Design
Behavioral Design
Behavioral design is the practice of shaping a product or environment so that one specific action becomes more likely.
AI & Engineering
Benchmark
A fixed task, dataset and scoring rule that many teams run, so their results can be compared on equal terms.
Psychology
Choice Overload
The drop in motivation and satisfaction that happens when there are too many options to choose from, even though more choice felt appealing beforehand.
Marketing
Curiosity Gap
The curiosity gap is the difference between what a reader knows about a topic and what they want to know, created on purpose by naming a topic and withholding one fact.
Psychology
Curse of Knowledge
Curse of knowledge is the difficulty an informed person has imagining what someone else does not already know.
Design
Gestalt Principles
Rules describing how people group what they see into whole shapes, using cues like closeness, likeness, alignment and enclosure.
Design
Information Architecture
Information architecture is the underlying structure of how a product's content is grouped, labeled, navigated, and searched.
Psychology
Intention-Behavior Gap
The difference between what people intend to do and what they actually do, measured as the share of intenders who do not act.
Design
Jakob's Law
Jakob's Law says people expect a new product to work like the other products they already use, because most of their experience comes from elsewhere.
Product
JTBD
A way to explain why people choose a product by the progress they want in a specific situation, not by who they are.
Psychology
Paradox of Choice
The finding that more options can reduce, rather than increase, a person's satisfaction with and motivation to make a choice.
Design
Progressive Disclosure
A design principle: show only the few options most people need first, and keep advanced or rarely used ones behind an explicit request such as an Advanced button.
Product
The Experimental Mindset
The experimental mindset treats a decision as a hypothesis to test with a small, reversible trial, so that a failed test counts as information rather than a loss.
Psychology
Theory of Mind
The ability to attribute beliefs, desires and intentions to other people and to predict what they will do from those mental states.
Psychology
Variable Reward
A payoff that changes in size or timing after an action, so people cannot predict it and tend to repeat the action more.
Psychology
Working Memory
The small amount of information a person can hold in mind and actively work with at once, roughly three to four chunks when rehearsal is prevented.
Psychology
Analysis Paralysis
A pattern where a person or team keeps gathering and comparing information instead of deciding, because each addition raises the felt risk of choosing wrong.
Psychology
Anchoring
The tendency for a number seen just before an estimate to pull that estimate toward it, even when the number is known to be irrelevant.
Design
Choice Architecture
The design of the setting in which people choose, where the order, grouping, defaults and wording of the options change what gets chosen even though the options themselves stay the same.
Business
Cost-Benefit Analysis
A method for deciding whether an action is worth its price by giving every cost and benefit a money value and comparing the totals.
Design
Digital Nudging
Using interface design elements to guide people's choices on screens, without removing any option or changing its price.
Design
Empty States
The version of a screen, list or panel shown when it has nothing to display, which should explain why and offer a next step.
Design
Fitts's Law
Fitts's Law predicts that the time to move a pointer to a target rises with the distance to it and falls with the target's size.
Psychology
Framing Effect
The tendency to choose differently between logically identical options depending on whether the outcome is described as a gain or a loss.
Design
Natural Mapping
Placing controls so that their position and movement match the things they change, so people can tell which control does what without a label.
Product
Personas
A persona is a fictional but realistic person, built from research, who represents a group of users and guides design decisions.
Psychology
Salience
Salience is how strongly an element differs from its surroundings, which decides what the eye and attention reach first.
Design
Skeleton Screen
A loading state that shows grey placeholder shapes in the layout of the coming page, then fills each one with real content as it arrives.
Psychology
Sunk Cost Fallacy
Letting money, time or effort you've already spent and can't get back decide what you do next.
Product
Triage
Triage is the recurring practice of sorting incoming bugs, tickets, or incidents by urgency and impact under time pressure, deciding what gets attention now, later, or never.
AI & Engineering
Agentic Workflow
A process in which a language model completes a task in several steps, planning, using tools, checking and revising its work.
Psychology
Availability Heuristic
A mental shortcut where people judge how frequent or likely something is by how easily examples come to mind, rather than by the real data.
AI & Engineering
CNN (Convolutional Neural Network)
A neural network that learns image features by sliding small reusable filters across the input, making it efficient at recognizing patterns like edges, shapes, and objects.
Business
Diminishing Returns
The law of diminishing returns says that as one input to production keeps increasing while every other input stays fixed, each additional unit adds less extra output than the unit before it, and eventually may add none at all.
AI & Engineering
Evaluation Set
Labelled data a machine learning model has never trained on, used to score how well it performs on new examples.
Marketing
Free
The zero-price effect: people treat a price of zero as a special price, so making an option free shifts choices far more than any other price cut of the same size.
AI & Engineering
Human Evaluation
Judging a system's output by asking people to rate, rank or mark errors in it, instead of computing a score automatically.
Design
Law of Similarity
The Law of Similarity is the Gestalt principle that elements sharing a visual trait, such as colour, shape or size, are seen as one related group.
Marketing
Perceived Value
A buyer's own judgment of what a product is worth, formed by weighing what they believe they receive against what they believe they give up.
Psychology
Planning Fallacy
The habit of expecting your own project to take less time and money than similar projects did, even when you know their record.

Every term worth knowing — a concept library for designers, PMs, engineers, founders, and tech leaders.