Interpolation
Working out the values between two known ones, such as the colour halfway between two colours.
See it in Image morph. Tap the picture to morph, then move Crossfade midpoint to change where the blend is half and half.
In plain words
Interpolation fills the gap between two values you already know. Halfway between 0 and 100 is 50, and a quarter of the way is 25. The same idea works for positions, sizes and colours. Animation uses it to draw the frames between two keyframes. Enlarging a picture uses it to guess the new pixels between the old ones.