This term refers to three instances in Cricket, which are :
1.    Dismissal of a batsman
2.    Pitch
3.    Stumps and Bails

Dismissal of a batsman :

The event of a batsman getting out is called as wicket. It is said that, “The batsman has lost his wicket”. When the batting team loses its 10 wickets, their innings ends. Whenever a bowler dismisses a batsman, it means that the bowler has taken the wicket and it adds up to his tally of wickets. The number of wickets by a bowler is the measure of performance of that bowler in the match.
 

The numbers of wickets, taken by a bowler are also calculated throughout his career. The highest wicket taking bowler in both Test  cricket and ODIs  is Muttiah Murlitharan with 792 wickets  in Test cricket and 515 wickets in ODIs, as on 6 July , 2010.

A batsman can lose his wicket, if his wicket is put down. This can happen if he is either Bowled, Stumped, Hit Wicket or Run Out. The wicket is put down if a bail is completely removed from the top of the stumps, or a stump is struck out of the ground by the ball, the strikers bat, the strikers person (or by any part of his clothing or equipment becoming detached from his person), a fielder (with his hand or arm) and providing that the ball is held in the hand or hands so used, or in the hand of the arm so used. The wicket is also put down if a fielder pulls a stump out of the ground in the same manner.

If one bail is off, removing the remaining bail or striking or pulling any of the three stumps out of the ground is sufficient to put the wicket down. A fielder may remake the wicket, if necessary, in order to put it down to have an opportunity of running out a batsman.
 If however both bails are off, a fielder must remove one of the three stumps out of the ground with the ball, or pull it out of the ground with a hand or arm, providing that the ball is held in the hand or hands so used, or in the hand of the arm so used.

Sometimes, the umpires may want to put away the bails, because of the strong winds, not allowing the bails to remain on top of the stumps. Here, the decision as to whether the wicket has been put down is one for the umpire concerned to decide.

Pitch :

The term wicket is also used for the pitch, although according to the Laws Of Cricket, this is incorrect, but it is in common usage and commonly understood by cricket followers. It is usually used to describe the nature of pitch like, “A Grassy Wicket”, “ Dry Wicket”, “Damp Wicket”.

 Stumps and Bails :

The wicket is one of the two sets of three stumps and two bails at either end of the pitch. The wicket is guarded by a batsman who, with his bat, attempts to prevent the ball from hitting the wicket. The size and shape of the wicket has changed several times over the years, but now its dimensions, positioning and placement is determined according to the Law 8 in Laws Of Cricket.

According to the Law 8 :

The wicket consists of three wooden stumps that are 28 inches (71 cm) tall. The stumps are placed along the batting crease with equal distances between each stump. They are positioned so they are 9 inches (23 cm) wide. Two wooden bails are placed on top of the stumps. The bails must not project more than 0.5 inches (1.3 cm) above the stumps, and must, for mens cricket, be 45⁄16 inches (10.95 cm) long. There are also specified lengths for the barrel and spigots of the bail. There are different specifications for the wickets and bails for junior cricket. The umpires may dispense with the bails if conditions are unfit (e.g., if it is windy they might fall off by themselves).