A batsman may challenge a decision of out, and the captain of the fielding side may challenge a not out decision. Teams are limited in the number of unsuccessful reviews per inning; if they have reached that limit they are permitted no further reviews. The umpires themselves can review run out, caught and no ball decisions, or whether a shot has scored a boundary. Only available in televised games, because the necessary equipment is provided by the broadcaster. Umpires call
Outcome of the Umpire Decision Review System if the third umpire finds the evidence inconclusive, or within the stated margin of error. The original umpires decision stands, but the review does not count towards the team limit on unsuccessful reviews. Uncapped
a player who has never played at the international level (see "cap (sense 2)".) Underarm
the action of bowling with the arm swinging from behind the body in a downswing arc and then releasing the ball on the up swing without bending the elbow. This type of bowling is now illegal in formal cricket, but commonly played in informal types of cricket. Compare with overarm. Under-spin (also back-spin)
backward rotation on the ball, causing it to decrease speed immediately after pitching. Unorthodox
1.  a shot played not in the accepted "textbook" manner, often with a degree of improvisation. 2.  a left arm spin bowler who spins the ball with their wrist in a similar manner to a right-arm leg-spin bowler. This imparts spin in the same direction as a right-handed off-spin bowler. See: Left-arm unorthodox spin.
Twelfth man
Traditionally, the first substitute player who fields when a member of the fielding side is injured. In Test matches, twelve players are named to a team prior to the match, with the final reduction to eleven occurring immediately prior to play commencing on the first day. This gives the captain some flexibility in team selection, dependent on the conditions (e.g. a spin bowler may be named to the team, but omitted if the captain feels that the pitch is not suitable for spin bowling). Twenty20 (or T20) 
a form of limited overs cricket in which each team has one innings with a maximum length of twenty overs. Two
batsmans call for a probable two runs, requiring their partner to commit to a fast turn. See No, Push, Yes, Wait. U
Umpire
An official who enforces the laws and adjudicates play. One umpire stands behind the wicket at the non-strikers end, while a second (usually) stands at square leg, with the positions alternating for each over. The two on-field umpires use a system of arm signals to indicate decisions to the players and the scorer. Televised matches usually also have a third umpire to adjudicate on replays and the umpire decision review system. Umpire Decision Review System (UDRS)
(or Decision Review System, DRS)Official system to challenge certain decisions made by an umpire. The third umpire then reviews the decision using various technological aids, such as slow motion television replays, ball tracking, a snickometer etc.