Slerp
Interpolating between two orientations along the shortest arc on the quaternion sphere, at a steady angular velocity the whole way.
Ken Shoemake introduced slerp (spherical linear interpolation) in the paper Animating Rotation with Quaternion Curves at SIGGRAPH 1985 — the same work that brought quaternions into computer graphics. Plain linear interpolation between two rotations gives uneven speed and can cut straight through the sphere; slerp instead follows the great circle so the turn happens at constant velocity.
In practice people often reach for nlerp — normalised linear interpolation — which is cheaper and nearly identical over short arcs, saving slerp for the cases where even speed really shows. It is the heart of how skeletal animation and other tweening blend between poses.