The rule for “offside” (always without an “s”) is a simple concept yet oh so subjective and chock-full of nuance. It basically aims to prevent what Americans call “cherry picking”, or skulking near the goal waiting for a pass and an easy opportunity to score.
IFAB (International Football Association Board) has codified the rules of association football (aka soccer) in its 200-page Laws of the Game. FIFA, the Fédération Internationale de Football Association (International Federation of Association Football), as the ruling body for association football, recognizes IFAB as having the sole jurisdiction for defining the Laws of the Game. IFAB’s Law 11 covers the “offside” infraction.
For this simple rule, IFAB uses three pages to explain that a player is guilty of the offside infraction when two conditions are met: (a) they are in the offside position, and (b) they become involved in active play. Therefore, simply being in an offside position is not an infraction.
According to Law 11, a player is in the offside position when “any part of the head, body or feet is in the opponents’ half (excluding the halfway line) and any part of the head, body or feet is nearer to the opponents’ goal line than both the ball and the second-last opponent” (the last opponent usually being the goalkeeper). Again, merely being in this position is not an infraction.
If, however, a player in the offside position becomes involved in active play, they have committed the dreaded offside infraction. While IFAD goes into detail about the finer points of “active play”, it boils down to this: if you are in the offside position and your team gains an advantage from it, you’re likely offside.
So simple, yet so complicated. When in doubt, ask yourself this question: was the goal too easy? If so, then it was probably offside.