In tennis, the term for a serve that the opponent cannot touch is called an ace. An ace occurs when the server hits the ball in such a way that it lands within the service box and bypasses the opponent, making it impossible for them to return it. This is a significant achievement for the server, reflecting their power and precision. Aces not only contribute directly to scoring but also serve as psychological advantages, instilling fear in the opponent and boosting the server's confidence during the match.