The term 'Tennis Umpire's Calls' refers to the verbal signals given by a tennis umpire to indicate the outcome of a point. The most common call is 'Lets', which is used to indicate that the ball was served in the correct manner and the point can be played. This call is usually followed by a hand gesture from the umpire, such as raising their arm or pointing to the server. Other calls include 'FAULT', 'OUT', and 'ACE'. These calls are used to indicate when a player has made an illegal move or when a player has won the point with a serve that cannot be returned.