Rigging
Also said as Rig, Inverse kinematics, IK, Forward kinematics, FK
Building the skeleton and the controls that move a character. With forward kinematics you rotate each joint in turn, from the shoulder down. With inverse kinematics you place the hand, and the solver works out the joints.
A good rig is what makes animating quick. IK also works at runtime, so feet land on real slopes and hands reach real door handles, things no pre-made animation can know about.
Say it
“Rig the legs with IK at runtime. Cast down from each hip, place the foot on what you hit, and lower the pelvis so the longer leg can still reach on stairs.”
Where you’ve seen it
Spore had to animate creatures that players had built themselves, so a 2008 SIGGRAPH paper describes retargeting its motions to any body at runtime, solving limbs by inverse kinematics. Rain World animates its creatures in code too, rather than from drawn frames.
Not to be confused with skinning
Rigging builds the bones and controls. Skinning sets how the surface bends with those bones. The word rigging often covers both.