In outdoor soccer, or outdoor football as it is known in most of the world, an offside foul is called when an offensive player, or attacking player, is passed the ball and there are not at least two opponents between him or her and the goal line. Usually, these are the goalkeeper and one other defender, but not necessarily. The penalty for an offside foul is that the other team is given possession of the ball.

The rule was created to prevent offensive players from "cherry picking" near the opponents' goal. Without the rule, offensive players could hover near their opponents' goal even when the play is on the other side of the field, with the hope of a long pass and an easy goal. The offside rule still allows for the chance to score.

It's also important to note that offside applies at the moment the ball is passed, not at the moment the ball is received. Therefore, if the offensive player that will receive the pass is "onside" at the time the ball is passed, but then runs behind the unsuspecting defender before receiving the pass, the receiving offensive player is not offsides.

To make things a little more difficult for the referee and the offensive team, defenders can play the "offside trap." The offside trap is a strategy where defenders try to draw an offside penalty by running forward right before a potential pass, thereby drawing the furthest forward offensive player offside. The offside trap can backfire however if the defenders do not get ahead of the receiving offensive player before the pass is made. If the receiving offensive player is level with the defenders moving forward at the moment of the pass, the player is onsides, often resulting in the receiving player being wide open.

There are some exceptions to the rule of offside. It cannot be called if the offensive players are on their own side of the field or if the ball is passed backward anywhere on the field. Nor is it valid on a throw-in, goal kick, or corner kick.

Offside may be spotted by either the center referee or by the sideline referees. If the center referee spots the offside he or she will blow his or her whistle to stop play. More commonly, a sideline referee will signal the offside by raising a red flag indicating to the center referee that he or she should blow his or her whistle to stop play. While the center referee doesn't have to blow his or her whistle upon the sideline referee's signal, the center referee usually will. Once offside is spotted and the center referee blows his or her whistle to stop play, he or she will place the ball at the point of the infraction. The opposing team will then get an indirect free kick from that position when offside is called in their favor.

Offside has always been a controversial rule, particularly because a referee has some freedom of interpretation, but also because it's often difficult to judge the player's positioning at the moment of a pass. The referee must determine if the circumstances were right to make an official offside call, even if the player was technically occupying an offside position. A referee has flexibility in ruling that the player seriously intended to receive the pass, was moving forward, and was significantly "interfering with play," as the rule book states. This means the attacker had a fair chance of scoring a goal in addition to being offside.