- Football over the line of scrimmage or otherwise ahead of the ball before the play has begun, and hence subject to penalty
- Ice Hockey moving into the attacking zone ahead of the puck
- Sports Illegally ahead of the ball or puck in the attacking zone.
- Football Illegally beyond the line of scrimmage when the ball is snapped or ahead of the ball when the ball is kicked on a kickoff.
(comparative more offside, superlative most offside)
- Precise usage varies between sports; singular offside is used more in association football, while the plural form offsides is, incorrectly, used by some in American football.