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.
The third umpire was conceptualized by former Sri Lankan Test cricketer, and current cricket writer Mahinda Wijesinghe  and debuted in Test cricket in november,1992 at Kingsmead, Durban for the South Africa vs. India series. Karl Liebenberg and Cyril Mitchley were TV umpires in this match. Sachin Tendulkar became the first batsman to be dismissed (run out) by using television replays in the second day of the Test scoring 11.
In the case of a run out or stumping, a batsman may be declared "out" if the wicket is 'put down'. The wicket is down if a bail is removed from the top of the stumps or a wicket is struck out of the ground, whether by the ball, the bat or the striker's person. If the batsman is caught between the two popping creases if the wicket is put down, the batsman is declared out. In most cases, the event occurs in a fraction of a second. If the field umpires are unable to accurately come to a decision on the dismissal of a batsman, the umpire requests the third umpire to ascertain whether the batsman had made it home. The third umpire then looks at various TV replays from different angles and comes to a conclusion by pressing the appropriate signal. Originally decisions were conveyed in traffic light style (a red light indicating a batsman's dismissal, a green light not out); it is now common practice to display the decision via the large screen scoreboard, if available. If the umpire is unsure if a batsman is out or not, due to lack of conclusive evidence, the usual procedure is to acquit the batsman, known in cricketing parlance as "the benefit of the doubt".
The third umpire may also be called upon if the on-field umpire cannot decide which batsman is out (i.e. they end up at the same end). An example of this was the Third Test between New Zealand and the West Indies in 2006.
A batsman is caught out if a fielder catches the ball on the fly. In some cases the fielder may catch the ball a few inches above ground level. If the umpire's vision is obscured or is unsure if the ball bounced before the fielder caught the ball, he can also refer the decision.
A six is scored if the batsman hits the ball directly beyond the perimeter of the field. In some cases the ball may bounce just a foot inside the boundary rope resulting in four runs. If the umpire needs to ascertain if it had been a 4 or a 6, he may consult the third umpire. Near the boundary, often a fielder may dive to save the ball from travelling beyond the boundary. If the fielder makes any simultaneous contact with the boundary and the cricket ball, 4 runs are declared. A third umpire may also be consulted in such a case.
Whenever a 3rd umpire decision is signalled by an on-field umpire. The 3rd umpire gets access to Instant Replay to make his decision. The instant replays are also available for the TV/Internet viewers. However, the live crowd does not get to witness it. Because in most of the cases, 3rd umpire decisions end up in a tight gap between out and not out. Live crowd will make their decision on the replay shown. However, the third umpire is required to follow guidelines which may turn the decision to the other side. This could lead to major problems among the crowd and disrupt the gameplay.
After Stuart Broad incident, in The Ashes 2013, ICC has started to take steps to give third umpire access to instant replays. This is regardless of being referred to by on-field umpires. By doing so, ICC wants to make sure any obvious mistakes are avoided in future.