What is a legal serve that the receiver fails to touch, winning the point outright? An ace. The server wins the point immediately when a correctly delivered serve lands in the service box and the receiver cannot touch it with the racket.
An ace is a special type of serve winner, but the terms are not identical. A serve can be a service winner if the receiver gets a racket to it but cannot control the return; an ace is generally recorded when the receiver does not touch the ball at all. A serve that lands outside the correct box is a fault, not an ace.
Aces are most common on fast courts, where the ball travels quickly and stays lower after the bounce. Players also use placement, spin, and disguise—not just raw speed—to prevent a return. A well-placed serve near the centre line or the outside corner can be just as effective as a very fast one.
The word “let” is another common mix-up. A service let usually occurs when a served ball clips the net and still lands in the correct service box; the serve is replayed, and no point is awarded. A “winner” is a broader term covering an untouchable shot during a rally.