Kampyle (Greek καμπύλη: a curve, the feminine of καμπύλος "bent"). The bend of a motion, read as a number. A design system declares its curves once as CSS strings — linear, ease-in-out, cubic-bezier(0.4, 0, 0.2, 1) — and the browser reads those for its own animations; this reads the same strings for the three places the browser is not animating: a pose derived on a stage, a frame of a film rendered with no document in the room, a value tweened where no browser stands at all. A timing string is read as four bezier numbers, the parameter is solved for x by forty steps of bisection with an early break at 1e-6, and the answer is read on y. The whole point is that a transition and its own motion under the same key bend THE SAME WAY, in both halves of a house about to be split in two. IT HOLDS NO TABLE: which curve a key means belongs to whoever declares it, and the one host surface is a single verb — give me the CSS string for this key — whose null answer is an honest one. AN UNKNOWN KEY IS A REFUSAL AS DATA, never a throw and never a silent straight line: four ways to fail, four plain sentences, no exception anywhere. An overshooting curve is TOLD APART rather than clamped, because a spring that anticipates below zero and overshoots above one is doing what a spring does, and flattening one quietly is how motion dies. Progress outside 0..1 is brought in rather than extrapolated. Zero imports, no DOM, no disk, no clock: a key and a number in, a number out, the same on every machine.
Greek kampyle (a curve), the feminine of kampylos (bent); the bend of a motion, read here as a number rather than drawn as a line.
A curve declared once and read the same way everywhere it is needed, so a transition and its own motion bend together rather than apart.