[ adj ]
illegally beyond a prescribed line or area or ahead of the ball or puck
<adj.all>
the touchdown was nullified because the left tackle was offside

offside \offside\ offsides \offsides\adj. (Sports)
Illegally beyond a prescribed line or area or ahead of the
ball or puck; -- in sports such as football or hockey; as,
the touchdown was nullified because the left tackle was
offside.
[WordNet 1.5]