Animation blending
Weighting two or more animation clips together by interpolating the bone transforms, so transitions come out smooth instead of snapping.
The simplest form is a crossfade: as a character goes from idle to walking, the walk clip's weight ramps from 0 to 1 over a couple of tenths of a second while the former ramps down. Rotations interpolate with quaternion slerp so joints don't fold the wrong way en route. A blend space goes further and mixes several clips continuously along a parameter — walk, jog and sprint blended by actual speed.
Additive blending layers one motion on top of another: an aim offset or a breathing loop is added to the base walk without replacing it. Engines expose all this as blend trees (Unity's Mecanim, Unreal's animation blueprints), and it is the glue that binds mocap clips and keyframes into coherent motion.