DEFINITIONS

At the start of a game all players are onside. As the match progresses players may find themselves in an offside position. Such players are then liable to be penalised until they become onside again.
In general play a player is offside if the player is in front of a team-mate who is carrying the ball, or in front of a team-mate who last played the ball.
Offside means that a player is temporarily out of the game. Such players are liable to be penalised if they take part in the game.
In general play, a player can be put onside either by an action of a team-mate or by an action of an opponent. However, the offside player cannot be put onside if the offside player interferes with play; or moves forward, towards the ball, or fails to move 10 metres away from the place where the ball lands.