In ice hockey, a play is offside if a player on the attacking team enters the offensive zone before the puck itself does, and then receives possession of the puck from the neutral zone. Simply put, the puck must not enter the attacking zone after attacking players. If a player on the attacking team is in the offensive zone before the puck, either an immediate offside occurs, or they must retreat to the neutral zone.

For determining offside, the position of the puck and players must be considered to be in the neutral or offensive zone. Usually, the puck enters or leaves the offensive zone when it completely crosses the blue line. A player is in the neutral zone if either skate is in the neutral zone or touches the imaginary plane extending upwards from the leading edge of the blue line; otherwise, he is in the offensive zone.

When an offside violation occurs, a linesman will stop play. To restart play, a faceoff is then held at the ice spot closest to the infraction, usually a neutral spot, or if there is a delayed penalty, at a spot in the defending zone of the defending team, which incurred the penalty. Even if the linesman erred in calling offside, the faceoff will still occur.