An offside (formerly off-side) rule is a part of many field sports, including most versions of football and ice hockey. Until recently field hockey also had a similar rule.
In general, such a rule makes it an offence to join the action from too far upfield and to gain an advantage by loitering in the vicinity of an opponent's goal in the hope of a long ball. It is generally believed that such loitering would damage the enjoyment of the game of both spectators and players.
The offside rule in field hockey was eliminated in 1996. There is a wide perception that it has improved the game and that the feared tactic of goal hanging has, in general, failed to materialise.
External links
Perceived effects of eliminating offside in Hockey (http://www.ncaa.org/news/1998/19981012/active/3534n10.html)
A player in an offside position is only committing an offside offence if, "at the moment the ball touches or is played by one of his team", the player is in the referee's opinion involved in active play by: interfering with play; interfering with an opponent; or gaining an advantage by being in that position.
Gaining an advantage by being in an offside position means playing a ball that rebounds to him off a post or crossbar or playing a ball that rebounds to him off an opponent having been in an offside position.
One of the best-known defenders to employ the offside trap was Billy McCracken of Newcastle United.