offside rule

Short definition from Wiktionary

Noun

1. 1
   The rule determining whether a player is in an offside position; that is, an illegal position ahead of the ball, puck, etc.
2. 2
   The rule determining whether a player is in an offside position; that is, an illegal position ahead of the ball, puck, etc: The prohibition of teammates closer to the opponents' goal than the person kicking or throwing the ball unless there are two (originally three) opponents at least as close to their goal line.
3. 3
   A system that gives right-of-way at an intersection to traffic approaching from the side on which one is not driving.
4. 4
   In some programming languages, the convention by which the indentation of a line of source code affects its meaning.