Blend tree
Also said as Blend space, Animation blending, Locomotion blending
A small graph that mixes animations by a number from the game. Feed in speed and it returns, say, 30% walk and 70% run, averaged joint by joint and kept in step, so there is no separate clip for every speed.
Without blending, a character snaps between clips and pops at every change of speed. It also means a few clips can cover thousands of in-between moves.
Say it
“Use a 1D blend tree on speed: idle at 0, walk at 1.5 m/s, run at 5 m/s, with foot phases synced. Then add a lean as a second axis, driven by turn rate.”
Where you’ve seen it
Unity's Animator has a component literally called a Blend Tree, and Unreal Engine's equivalent is called a Blend Space.
Not to be confused with state machine
A state machine picks which thing the character is doing and handles the switch. A blend tree mixes clips continuously within one of those states.