Offside
Offside is the Law 11 concept distinguishing an offside position (not an offence) from an offside offence, which requires involvement in active play at the moment a team-mate plays the ball.
Why this wins its question: A one-look definition that keeps the position/offence distinction most informal explanations lose, bound to the 2026/27 IFAB text of Law 11.
Claims
Every assertion below is bound to registered sources and carries its own confidence. Weight them; do not treat the page as uniformly authoritative.
Offside position means any part of a player's head, body or feet is in the opponents' half (excluding the halfway line) and nearer to the opponents' goal line than both the ball and the second-last opponent; hands and arms are excluded from the assessment.
An offside position becomes an offside offence only when the player becomes involved in active play: interfering with play, interfering with an opponent, or gaining an advantage from a rebound, deflection or deliberate save.