Morph target
An animation technique that deforms a mesh by blending between stored snapshots of its vertex positions — the standard trick for facial expressions.
Also called blend shapes or shape keys. You store a set of vertex positions for each target — a smile, a raised eyebrow — and the runtime applies a weighted blend of the deltas from a base mesh. Weights between 0 and 1 let expressions overlap, so a half smile and a raised eyebrow can share a single frame.
The method shines where a skeleton is clumsy, above all in faces, where muscles pull skin in patterns bones capture poorly. In practice it is combined with skeletal animation and vertex skinning — bones for the body, morph targets for the face. The cost is memory: each target is essentially a full copy of the mesh's vertex positions.