In tennis, what is a half-volley?

A half volley is hitting the ball right after it bounces. Technically it's not a volley, since you're not taking it out of the air, but many of the techniques involved are similar. Often a player at the net is forced into making half volleys by opponents who aim at their feet.

It is usually very difficult to hit aggressively because you have to hit the ball over the net from such a low contact point. It takes great skill to make good half volleys. Some pros who are exceptional at it are Sampras and McEnroe, to name a few.

A half volley is when you meet/hit the ball as soon as the ball bounces off the surface, usually while you are approaching the net.

a half-volley is hit right after the ball hits the ground. its more like a scoop up. a volley is directly out of the air, and its more like a punch

The timing on overheads is always tough. The ball is usually dropping from a good height, so it has time to accelerate. The best thing to do is have someone feed you many overheads, every day. There's just no substitution when trying to get your timing down. No magic tip or formula to it. On volleys try to close to the net. Closer you get, less errors you will make. There should be little to no backswing on either side. During the points, keep your racquet at the volley ready position and think like the ball is going to come your way every time someone hits it. Split step at the exact moment your opponent(s) strikes the ball. This means a small hop, feet shoulder length apart, and weight moving forward. Stay on your toes, and once again, assume every ball is coming your way. The accuracy on the volleys will come with practice. Keep yourself balanced, keep the racquet out in front, and use short punching motions like you're high fiving someone. Less backswing you use, the better your volleys will be.