Blend tree
A node graph that smoothly mixes several animation clips by weight — walking easing into running as speed rises — instead of hard-cutting between them.
The building block of Unity's Mecanim and Unreal's blend spaces. A 1D blend takes one parameter (speed) and weighs two clips together; a 2D blend weighs four or more by both direction and speed. The mixing happens per bone by interpolating poses, usually with nlerp on the rotations.
The tree then feeds the animation state machine, which decides which clips are active at all. Without this smooth transition you get popping — the jolt that appears when a character switches animation mid-motion.