Glossary

Acknowledgment — The first visible response to an input, delivered on input start within the response budget.

Bounce — The perceptual spring parameter controlling overshoot. 0 is critically damped (no overshoot); positive values overshoot; negative values are overdamped. Set via the character token.

Channel — The classification of a value. Spatial channels (position, size, scale, rotation, shape) default to springs; effect channels (opacity, color, blur, shadow) default to tweens.

Character — Token scale for how much a spring overshoots: flat, smooth, lively, bouncy.

Choreography — The rules governing how multiple motions relate in space and time: origins, relationships, ordering, staggering, symmetry.

Commit — The point at which an interaction's outcome is performed (a press activates; a drag decides to dismiss).

Decay — A motion that coasts from current velocity to rest, slowing continuously, with no target. Used for momentum and for projection.

Direction locking — Deciding a drag's axis at the hysteresis threshold and holding it, so perpendicular movement goes to another owner (usually scroll).

Effect channel — See Channel.

Environment — Context that affects resolution: input modality, hover availability, device capability, performance headroom.

Essential motion — Motion that is the information or function, so removing it would change what the content means or does. Direct manipulation is essential. Exempt from reduced-motion substitution.

Frequency — Token scale for how often a person encounters a motion: constant, frequent, occasional, rare. Caps tempo and character.

Gesture lifecycle — The shared sequence of states every interaction follows: Rest, Engaged, Tracking, Released, Committed, Settling, Cancelled.

Grab offset — The distance between where the input touched an object and the object's own reference point, preserved throughout an interaction.

Hand-off — The transfer of a value's ownership between an interaction and a motion (or between two motions), always carrying presentation value and velocity.

Hysteresis — A small movement threshold an input must cross before it commits to being a drag.

Intent — A declaration of what a moment means (acknowledge, reveal, navigate…), which resolves to purpose, channels, motion, tokens, choreography, and reduced behavior.

Interaction — A person directly controlling values through input. One of the two kinds of value owner.

Interruption — New input or a new target arriving while a motion is in flight. Always allowed; always continuous.

Motion — A behavior that moves a value without a person's direct control: spring, decay, or tween. One of the two kinds of value owner.

Origin — The place a revealed, concealed, navigated, or morphing element is spatially connected to: a trigger, an edge, itself, or a shared element.

Owner — The single motion or interaction controlling a value at a given instant.

Perceptually complete — The moment a motion looks finished. Follow-up UI may react here. Contrast with settled.

Preference — What the person has asked for, chiefly motion preference (full or reduced). Resolved last; never overridden.

Presentation value — What a value is on screen right now, as opposed to its target.

Projection — Using release velocity and decay to predict where a gesture was heading, and choosing the outcome from that prediction.

Purpose — Why a motion exists: feedback, orientation, state, attention, or delight. Every motion has exactly one primary purpose.

Reduced behavior — The substitution an intent uses when the person prefers reduced motion. Required for every intent.

Resistance — Progressive damping of movement past a boundary (rubber-banding).

Resolution — The fixed-order process (intent → author → frequency → environment → preference) that turns an intent into concrete motions. Each step can only reduce.

Retarget — Changing a motion's target while it's in flight, without resetting its velocity.

Settled — A value within a negligible threshold of its target with effectively zero velocity. Ownership and resources are released here. Nothing a person waits on is tied to settling.

Spatial channel — See Channel.

Spring — A motion that moves a value toward a target, respecting current velocity, described by perceptual duration and bounce. The default for spatial channels.

Stagger — Small, capped offsets between the starts of motions in a group. Decorative; never delays interactivity.

Tempo — Token scale for how long a motion feels: instant, swift, quick, standard, relaxed, deliberate.

Tween — A motion along a fixed easing curve over a fixed duration. The default for effect channels.

Value — Any continuously changing quantity that affects what's on screen. Has a presentation value, velocity, target, and owner.

Velocity — How fast and in which direction a value is changing. Always carried through hand-offs.