Definition from Wiktionary, the free dictionary
Audio (US) (file)
- (sports) In an illegal position ahead of the ball
- (US) To the side of the road, past the curb and sidewalk, e.g. an offside diner (restaurant.)
- (bridge) Unfavourably located, from the point of view of the player taking a finesse.
Usage notes 
- Precise usage varies between sports; singular offside is used more in football, while the plural form offsides is used in american football.
in an illegal position ahead of the ball
offside (plural offsides)
- (sports) An offside play
- (UK) the right side of a road vehicle when facing in the same direction as the vehicle
- (roadside): nearside