Glossary

The ease of performing a behavior right now — one of the three factors in the Fogg Behavior Model, measured across time, money, effort, brain cycles, social deviance, and routine.

Designing products that people with disabilities can perceive, operate, and understand. Codified for the web in the WCAG standard.

The curved threshold in the Fogg Behavior Model separating behaviors that fire from those that don’t, plotted on motivation against ability.

Interfaces people find beautiful are perceived as easier to use — polish buys patience and trust, and can mask real usability problems.

What an object’s properties allow a person to do with it — a handle affords pulling, a flat plate affords pushing. Popularized in design by Don Norman.

The desensitization that sets in when people receive frequent alerts, most of them false or unimportant, so that they ignore or silence alerts, including critical ones. Also called alert fatigue.

The tendency to prefer options with known probabilities of a good outcome over options with unknown probabilities, even when the unknown option is no worse. Described by Daniel Ellsberg in 1961.

The tendency to rely too heavily on the first number or piece of information encountered when making a judgment, and to adjust away from it too little, even when it is irrelevant.

The proportion between an image’s width and its height, written width:height, such as 16:9. Fitting media to a frame of another ratio means cropping it, adding bars, or stretching it.

The tendency of website visitors to ignore page elements that look like advertisements, sit near ads, or appear in places where ads usually go. Named in 1998 by Benway and Lane.

An invisible set of evenly spaced horizontal lines that text and elements align to, keeping vertical rhythm consistent across columns.

A navigation trail showing where the current page sits in the site hierarchy, letting users jump back up any level.

An eye-scanning behavior where readers skip the first words of each line when successive lines begin with the same word.

The classical rules for placing a text block on a page: block in the page’s proportion, inner margin narrowest, foot deepest, in the ratio 2:3:4:6.

The failure to notice large changes to a scene when they happen during a brief interruption, such as a blink, a cut, or a page reload, or while attention is elsewhere.

Visual elements in charts that are not needed to understand the data or that distract from it, such as heavy gridlines, 3-D effects, and decorative images. Edward Tufte coined the term in 1983.

Never remove something whose purpose you don’t understand — investigate the reason it exists first, then decide.

The design of how choices are presented, including the number of options, their order and description, and the default, which influences what people choose. The term comes from Thaler and Sunstein’s Nudge (2008).

Grouping individual pieces of information into larger meaningful units so more fits in working memory — the practical core of Miller’s Law.

The total mental effort a task demands of working memory. Good interfaces reduce it; every extra choice, step, or inconsistency raises it.

Models that describe how a person’s understanding of an information need interacts with the way a system organizes information. They include berrypicking, information foraging, anomalous states of knowledge, and exploratory search.

The story a user builds in their head about how a system works. Design succeeds when the interface teaches an accurate one.

Nielsen’s heuristic: follow platform and industry conventions so users never wonder whether different words or actions mean the same thing.

Limits that prevent wrong actions before they happen — a plug that only fits one way, a form field that only accepts digits.

Organizations design systems that mirror their own communication structures. Your org chart ships in your product.

The phenomenon of features designed for disabled people, such as curb cuts and captions, being used and valued by a much wider group than they were made for.

Once you know something, you can’t simulate not knowing it — why experts write confusing docs and ship jargon.

An interface deliberately designed to push users toward choices they wouldn’t otherwise make — the manipulative flip side of behavior design.

Encoding data as visual form — position, length, angle, color — so patterns and comparisons can be read at a glance.

The declining quality of decisions after a long run of choices. Interfaces that demand many small decisions burn it fast.

The decline in the quality of decisions after a long session of making them. Worn-out deciders tend to avoid trade-offs, postpone choices, accept defaults, and act more impulsively.

The tendency to accept a preselected option rather than choose another, driven by effort, loss aversion, and the endorsement a default implies.

The tendency to spend money more readily when it is held in small units than when the same amount is held in one large unit. Named by Raghubir and Srivastava in 2009.

How easily users can find out what actions are possible and where — determined by visibility, signifiers, and feedback.

Productivity soars when system response stays under about 400 milliseconds — the user and computer keep pace with each other.

An optical illusion of relative size: two identical circles look different sizes when one is surrounded by large circles and the other by small ones. Also called Titchener circles.

Ben Shneiderman’s eight rules of interface design: consistency, universal usability, feedback, closure, error prevention, reversal, control, and low memory load.

Nielsen’s heuristic: eliminate error-prone conditions or catch them before the user commits, rather than relying on good error messages.

How well people can understand why an AI system produced a given result. Explainable AI (XAI) methods, such as showing which inputs drove a prediction, help people check, contest, and calibrate their trust in automated decisions.

The predictable paths eyes take across a page — F, Z, layer-cake, spotted — documented through decades of eye-tracking research.

The most common reading pattern on text-heavy pages: two horizontal sweeps near the top, then a vertical scan down the left edge.

The system’s immediate, visible response to a user action — the confirmation that something happened and what it was.

The time to acquire a target is a function of its distance and size. Bigger, closer targets are faster to hit.

Behavior happens when motivation, ability, and a prompt converge at the same moment: B = M · A · P.

A complex system that works has invariably evolved from a simple system that worked. Complex systems designed from scratch don’t work.

Rules describing how perception groups visual elements — by proximity, similarity, closure, continuity, and figure-ground relationships.

The proportion 1 : 1.618, long used to derive layout and typographic scale relationships that feel naturally balanced.

When a measure becomes a target, it ceases to be a good measure. Metrics optimized directly get gamed.

The gap between what a user wants to do and how the interface lets them do it. Its mirror, the gulf of evaluation, is the gap in understanding what happened.

On evenly weighted layouts, attention flows diagonally from top-left to bottom-right, favoring the primary optical area and terminal area.

Inspecting an interface against a short list of usability principles — a fast, expert-driven complement to user testing.

Decision time grows logarithmically with the number of choices. Fewer, better-organized options mean faster decisions.

It always takes longer than you expect, even when you take into account Hofstadter’s Law.

Nir Eyal’s four-phase loop — trigger, action, variable reward, investment — describing how products build habits.

A system that keeps human judgment inside an automated decision: the machine proposes and a person approves, supervises, or corrects. The related postures are on the loop and out of the loop.

The cues — labels, links, snippets — users sniff to judge whether a path will lead to what they want. Strong scent keeps them moving; weak scent makes them pogo-stick.

Users spend most of their time on other sites, so they expect yours to work the same way as the ones they already know.

Scanning only the headings and subheadings of a page, skipping the body text between them — the reward for good heading hierarchy.

The idea that the future life expectancy of a non-perishable thing, such as a technology or an idea, is proportional to its current age: the longer it has lasted, the longer it is likely to last.

The width of a line of text, usually counted in characters per line. For a single column of body text, 45 to 75 characters is generally considered comfortable. Also called the measure.

The section-level menu showing a page’s siblings and children, complementing the site-wide global navigation.

The relationship between controls and their effects. Natural mapping — stove knobs arranged like the burners — needs no labels.

What a user believes about how a system works, built from every similar system they’ve used before. Interfaces fail when they fight it.

Eighteen design guidelines for AI-infused products, published by Microsoft researchers at CHI 2019 and grouped by when they apply: initially, during interaction, when the system is wrong, and over time. Distilled from more than 150 existing recommendations and tested by 49 practitioners against 20 products.

Working memory holds about seven items, plus or minus two. Chunking is how interfaces respect the limit.

A grid with both columns and rows, creating repeatable modules that give complex layouts consistent structure.

The drive to act — one of the three Fogg factors, fluctuating through pleasure and pain, hope and fear, acceptance and rejection.

An optical illusion in which lines of equal length appear different because of the arrowheads or tails at their ends. Franz Carl Müller-Lyer devised it in 1889.

The recurring structures — global nav, local nav, breadcrumbs, search, site maps — that let users move through an information space.

Jakob Nielsen’s ten general principles for interaction design, the most widely used checklist in usability evaluation.

An intervention that changes how a choice is presented to make one option more likely, without forbidding options or significantly changing incentives, and that stays easy to avoid. Popularized by Thaler and Sunstein’s Nudge (2008).

A new user’s first guided experience with a product. Mostly an ability problem: cut steps, pre-fill, and show the first win fast.

The finding that supervising an automated task leaves a person slower and less able to take over than doing the task themselves, through fading situation awareness, decaying vigilance, and unused manual skill.

More options attract attention but depress action and satisfaction — Barry Schwartz’s extension of choice-overload research.

Roughly 80 percent of effects come from 20 percent of causes. In products: most usage concentrates in a few features.

People judge an experience by its most intense moment and its ending, not its average — design the peak and stick the landing.

The tendency for pictures to be remembered better than words. It is usually explained by Allan Paivio’s dual-coding theory: pictures are stored both as images and as verbal labels.

Be liberal in what you accept, conservative in what you send. Interfaces should tolerate messy input and return clean output.

Task time drops as a power function of repetitions — fast gains early, diminishing gains forever after.

The principle that a component of a system should behave the way most of its users expect, so that it does not surprise them. Also called the principle of least surprise.

A security principle stating that every user, process, or program should have only the access needed for its legitimate purpose. In design, it means requesting permissions in context and making limited access the default.

The subjective ease with which information is processed. Fluent information tends to be judged more true, familiar, likable, and trustworthy, regardless of its content. Also called cognitive fluency.

Showing only the options needed now and revealing the rest on demand, keeping each decision small.

The signal that says “do it now” — the third Fogg factor. Without one, even motivated and able users don’t act.

Dieter Rams’ criteria for good design — innovative, useful, aesthetic, understandable, unobtrusive, honest, long-lasting, thorough, environmentally friendly, and as little design as possible.

Nielsen’s heuristic: show options rather than making users remember them. Menus beat command lines for occasional tasks.

The composition heuristic that groups of three or five read as one deliberate set with a natural center, while even counts split into pairs.

Dividing a frame into a 3 by 3 grid and placing key elements on the lines and intersections for natural visual balance.

Choosing the first option that meets an acceptability threshold rather than searching for the best. Herbert A. Simon coined the term in 1956 from satisfy and suffice.

The first and last items in a series are recalled best; the middle sags — position is a memory decision.

The perceivable mark that communicates an affordance — the label, icon, or shading that says “this is clickable.”

The proportion 1 + √2, about 2.414, the positive solution of x² = 2x + 1. Removing two squares from a silver rectangle leaves another silver rectangle. Also called the silver mean.

A placeholder layout of grey blocks shown while content loads — and the visual language every illustration on this site is drawn in.

Pre-selecting the choice most users want, converting a decision into a confirmation.

The tendency to copy what others do when unsure how to act, on the assumption that they know better. Robert Cialdini coined the term in Influence (1984). Also called informational social influence.

Scanning that skips around a page fixating on visually distinct elements — links, numbers, buttons — rather than reading in order.

Time to steer through a constrained path grows with its length and shrinks with its width — the law of tunnels, menus, and sliders.

The delay and error cost that appears when two signals about the same thing carry conflicting meanings, such as a label and its color. Named for the 1935 experiment in which naming the ink color of a mismatched color word proved markedly slower.

The error of drawing conclusions only from the cases that passed a selection process, such as active users or successful companies, while ignoring those that were filtered out. Also called survivor bias.

Every application has an irreducible amount of complexity. The only question is who has to deal with it — the user or the designer.

BJ Fogg’s method of building behavior by shrinking it until it’s almost effortless, anchoring it to a routine, and celebrating immediately.

Bruce Tognazzini’s foundational rules of interaction design, spanning anticipation, autonomy, consistency, and latency.

A heuristic evaluation framework from Microsoft, developed by Michael Medlock and Steve Herbst, that pairs nine tenets of good interface design with the traps that violate them. Every trap maps to exactly one tenet.

The handful of symbols users recognize without labels — search, home, cart, settings — earned through decades of consistent placement.

Nielsen’s heuristic: provide clearly marked exits — undo, redo, cancel — so mistakes are cheap to escape.

Nielsen’s first heuristic: keep users informed about what’s happening through timely, appropriate feedback.

Arranging elements by size, weight, color, and position so the eye meets them in order of importance.

A perceptual task in which people scan for a target among distractors. A target with a unique feature pops out; one that shares features with the distractors must be found item by item.

When one item differs from its neighbors, it’s the one remembered — the isolation effect behind every accent color.

The Web Content Accessibility Guidelines — the W3C standard organized around four principles: perceivable, operable, understandable, robust.

Perceived change is relative to the starting magnitude — a change users notice at small scale disappears at large scale.

The empty space around and between elements. Not wasted space — it groups, separates, and gives hierarchy room to work.

Performance rises with arousal to a point, then falls — moderate pressure helps, high pressure hurts, especially on hard tasks.

The scanning path on sparse layouts: across the top, diagonally down to the lower left, then across the bottom — where the call to action belongs.

Unfinished tasks are remembered better than finished ones — the open loop behind progress bars and completion meters.

The observation that when items are ranked by how often they are used, frequency falls roughly in inverse proportion to rank: the second item is used about half as often as the first. Named for George Kingsley Zipf, who described it in language in 1935 and generalised it in 1949.