"Offside (sometimes known as offsides) is a law in association football which states that if a player is in an offside position when the ball is touched or played by a team-mate, the player may not become actively involved in the play. A player is in an offside position when closer to the opponent's goal line than both the ball and the second-to-last defender (which is usually the last outfield player), and also in the opponent's half of the pitch. "Offside position" is a matter of fact, whereas committing an "offside offence" occurs when a player is "actively involved" and is subject to the interpretation of the referee. Goals scored after committing an offside offence are nullified if caught by the referee." [Offside (association football). Wikipedia]
The diagram example "Association football (soccer) offside" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Football solution from the Sport area of ConceptDraw Solution Park.