A serve that the receiver cannot touch is called an ace.
An ace is a legal serve that lands in the correct service box and is not touched by the receiver. Because the receiver makes no contact, the server wins the point immediately. A well-placed ace may be hit with great pace, but accuracy, disguise, spin, and placement can be equally important.
A serve can still be an ace without being the fastest serve in the match. Players often target the corners, the receiver's body, or a direction suggested by their toss and motion. An ace is recorded whether it comes on a first serve or a second serve, although second-serve aces are much less common because players usually reduce risk.
An ace is different from a service winner, which is a serve the receiver touches but cannot return effectively. It is also different from a double fault, which occurs when both service attempts miss; a let allows the serve to be replayed, while a fault is an unsuccessful service attempt.