soccer

Offside is Law XI in the publication Laws of the Game written by the IFAB and published by FIFA. The law states that, if a player is in an offside position when the ball is touched or played by a teammate, he may not become actively involved in the play. It is not an offense to merely be in an offside position. It has been noted that modifying the offside rule would be one way to increase scoring opportunities in football.

player is in an offside position if he is in his opponents' half of the field (pitch) and is closer to his opponents' goal line than both the ball and all but zero or one of his opponents. Put another way, an offside position has three components, all of which must be satisfied for the player to be in the offside position: First, the player must be on the opposing team's half of the field. Second, the player must be in front of the ball. And third, there must be fewer than two opposing players between him and the opposing goal line, with the goalkeeper counting as an opposing player for these purposes.
The 2005 edition of the Laws of the Game included a new International Football Association Board decision that stated being "nearer to an opponent's goal line" meant that "any part of his head, body or feet is nearer to his opponents' goal line than both the ball and the second-last opponent (the last opponent typically being the goalkeeper). The arms are not included in this definition."[2] This is taken to mean that any part of the attacking player named in this decision has to be past the part of the second-last defender closest to his goal line (excluding the arms) and past the part of the ball closest to the defenders' goal line.
It is not necessary that the goalkeeper be one of the last two opponents. It should be noted that any attacker that is even with or behind the ball is not in an offside position and may never be sanctioned for an offside offense. Regardless of position, there is no offside offense if a player receives the ball directly from a goal kick, corner kick, or throw-in.
IFAB has clarified in the 2009-2010 Laws of the Game that a player temporarily off the field of play is considered to be ON the boundary line at the point that he crossed over the boundary line.