Tween
Also said as Easing, In-betweening, Interpolation curve
Moving a value from A to B over a set time along a curve, so you only state the start, the end and the feel. The word comes from animators drawing the frames in between.
Almost everything that moves without physics is a tween: menus, health bars, score pops, doors. The curve is what makes it feel snappy or heavy, and a linear tween looks like a machine.
Say it
“Tween the health bar down over 250 ms with an ease-out, and have a white ghost bar follow it after a 400 ms delay so the player sees how much they just lost.”
Where you’ve seen it
Flash put the word in front of a generation of animators: set two keyframes, ask for a motion tween, and it fills in the rest.
Not to be confused with physics
A tween knows exactly where it will end up and when, so it cannot be pushed off course. Physics only knows the next step, and it can.