Definition: Taken from the ITTF Handbook for Match Officials
4.2.1 For each match there is an umpire, whose primary duty is to decide the result of each rally. In principle, the umpire has no discretionary powers, but he is required to exercise judgment in applying some laws and regulations, such as deciding whether a rally should be a let because a player’s service or return may have been affected by circumstances outside the player’s control, or whether a player’s behavior is acceptable.
4.2.2 Where the umpire is officiating alone, his is the final decision on all questions of fact that arise during a match, including decisions on all edge balls and on all aspects of service. In these circumstances he is directly responsible also for timing the duration of play, but when the expedite system is in operation he is assisted by another official who acts as stroke counter.
4.2.3 Although the umpire is obliged to accept certain decisions made by other match officials he is entitled to seek an explanation if he believes that an official has made a decision that is outside his jurisdiction. If, on enquiry, he finds that an official has acted in this way he can overrule the decision wrongly made by that official, either by reversing it or, more usually, by declaring the rally a let.
4.2.5 The players are under the jurisdiction of the umpire from the time at which they arrive at the playing area until the time at which they leave it.