sport The mistake of occupying an illegal position on the playing field (in football, soccer, ice hockey, field hockey, etc.).

Roget 238: dextrality^†; right, right hand; dexter, offside, starboard.   

Illegally beyond a prescribed line or area or ahead of the ball or puck:
— The touchdown was nullified because the left tackle was offside.

synonym: offsides.

Illegally in advance of the ball or puck.