In many team sports, defense (American spelling) or defence (Commonwealth spelling) is the action of preventing an opponent from scoring. The term may also refer to the tactics involved in defense, or a sub-team whose primary responsibility is defense. Similarly, a defense player or defender is a player who is generally charged with preventing the other team's forwards from being able to bear down directly on their own team's goalkeeper or goaltender. Such positions exist in association football, ice hockey, water polo and many other sports.
By sport
Field sports
American football
Association football
Australian rules football
Bat-and-ball sports
In bat-and-ball sports, the defending team is in the field, while the offensive team sends only a few players into the field to try to score at a time. These sports generally involve a member of the defense throwing the ball to a member of the offensive team, who then tries to hit it and run to various safe areas of the field to score points. Failing to hit the ball properly, or failing to reach the safe areas before the ball is used against the offensive player, can result in that player being left unable to score.
Baseball
Baseball is unlike most other competitive sports in that the defense is given control of the ball. Additionally, the number of players on the field at any given time is lopsided in favor of the defense which always has nine players on the field; the offense has between one and four.
Historically, each player on the team had a role both on offense and defense. Most amateur and professional leagues have adopted the designated hitter (DH) rule introduced by the American League of Major League Baseball. In leagues that use the DH, pitchers usually do not bat, and the DH bats in lieu of the pitcher and does not play a defensive position. The main league that has not adopted the DH rule is the Central League of Nippon Professional Baseball.
Each play starts with the ball in the hands of the pitcher, whose job as a member of the defense is to use his skills to somehow prevent the batter from reaching base. The pitcher throws the ball toward the catcher, who must catch the pitched ball if it is not hit by the batter. In each half-inning, the defense attempts to force three outs.
There are three basic ways in which an out can occur: 1.) If three strikes are recorded against the batter, 2.) if a ball hit by a batter is caught by a defensive player before it hits the ground, or 3.) if a runner who is between bases or has not reached a base to which he is forced is put out by a defensive player in possession of the ball.
If the batter manages to hit the ball, all nine defensive players become active and use the ball in attempting to prevent the batter from reaching base and runners already on base from advancing or scoring. while the offense is busy attempting to move runners around the baseball diamond toward home plate, the defense uses the ball in various ways to achieve outs.
If the defense forces three outs, their team is moved into the offensive role. The exception is if it is the ninth or an extra inning and they are ahead, in which case, the game ends and the defensive team wins.
Cricket
In cricket, the fielding team is the defense, while the batting team is the offense; the batting team can only put two players on the field at a time. By getting the batting team's batsmen out before they can hit the ball to the boundary or run between the two batsmen's grounds, the fielding team can prevent the batting team from scoring points. Players can be gotten out if they hit the ball in the air and it is caught by a defensive player before touching the ground, or if the ball hits a batsman's wicket, either when delivered by the bowler or when the batsman is not in his ground. By getting all but one of the batting team's players out, the fielding team ends their opponent's scoring turn, and may begin its own scoring turn next (though it can choose not to, in the case of the follow-on).