Wiktionary
n. (context cricket English) A match official who uses TV replays to assist the two umpires in the case of close decisions.
Wikipedia
In international cricket matches the third umpire (or TV Umpire) is an off-field umpire who makes the final decision in questions referred to him by the two on-field umpires. Television replays are available to the third umpire to assist him in coming to a decision. An on-field umpire can, at his own discretion, use a radio link to refer any close decision concerning dismissals (catches, runouts or stumpings) or boundaries to the third umpire.
Usage examples of "third umpire".
For the absolute power which establishes or saves them may also oppress or exhaust them, there is a gradual substitution of differentiated powers, held together through the mediation of a third umpire, caused by reciprocal dependence and an which is common to both.