Blend space
Continuously mixes several animation clips along one or two parameter axes — such as speed or direction — rather than cutting between discrete states.
Where a blend tree nests fixed blend nodes, a blend space maps a 1D or 2D input (say velocity on X, turn rate on Y) onto a weighted mix of sample clips positioned at coordinates within that space. Barycentric weights from the nearest samples let a character moving at 3 m/s while turning blend walk, run and strafe smoothly. Unreal Engine made "Blend Space" a first-class asset; Unity offers the same idea as 2D blend trees.
The canonical use is locomotion — a diamond of idle/walk/run/sprint samples driven by a single speed value. It removes the visible pops you otherwise get when a state snaps mid-stride.