What is a half volley in tennis?

It usually happens when you're caught in the middle of the court moving forward. The ball is rising off the court, so a short, firm stroke with an open face lifts it back over the net.

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

Questions

Is a half volley a volley?

Not by the rules: the ball has bounced. It's played like a volley, with a short block.