A serve that lands in the service box and is not touched by the returner's racquet. Pure serving dominance. Big servers can rack up 20+ aces in a match. Isner vs. Mahut at Wimbledon 2010 produced 216 combined aces over three days. An ace on match point is one of the most satisfying ways to close out a match.
"140 mph ace down the T. The returner didn't even move. That's his fifteenth ace of the match."