Squash and stretch
A shape flattens on impact and pulls long at speed, and keeps its volume while doing it. It is the first of Disney's twelve principles of animation.
It says what a thing is made of, and how hard it landed, without a single word. A square that squashes has weight and character.
Say it
“On landing, squash the character to 70% height and 130% width and spring it back in 150 ms. Keep its area constant.”
Where you’ve seen it
Celeste squashes Madeline's sprite on every landing and stretches it on every jump.
Not to be confused with scaling
Scaling changes volume. Squash and stretch trades width for height, so the thing feels like one lump of the same material.