What is an overhead in tennis?

It uses a shortened version of the serve's motion. The hard part is the footwork: turning sideways and moving under the ball early so it drops in front of you.

A tour overhead, instant 1 of 8
TRACED · A TOUR OVERHEAD
A tour overhead: eight instants traced from the footage, frame for frame. Hover or tap to flip.

Questions

Is an overhead the same as a serve?

It uses the same chain with a shorter backswing, and you hit it on the move.