In tennis, a winner occurs when a player is unable to touch the ball with their racquet before it bounces twice during a match. This results in a point for the opponent who hit it, and is deemed a winner for them.
Winners can occur many times throughout tennis matches. A winner does not happen on every point in tennis, despite a point being won by a player after each volley. If a player manages to hit the ball with their racquet in return, but either sends it out of play or into the net, it is not counted as a winner for the opponent, even though they are awarded the point.
Winners can sometimes occur on serves as well. A winner that happens on a serve is most commonly referred to as an 'ace', but it technically counts as a winner, too, as long as the opponent was unable to touch the ball with their racquet before it bounces twice. However, not all aces are winners, as a serve could be hit by the opponent either out of play or into the net.
This image depicts tennis player Serena Williams attempting to volley, but failing to reach the ball, resulting in a winner for her opponent.