In ice hockey, a play is offside if a player on the attacking team does not control the puck and is in the attacking or offensive zone when a different attacking player causes the puck to enter the offensive zone, until either the puck or all attacking players leave the offensive zone. Simply put, for a play to not be offside, the puck must enter the attacking zone before attacking players, and the offensive team cannot play the puck when offside.

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, they are in the offensive zone.

Being offside is generally not permitted; under some rule sets, an offside violation occurs as soon as a play goes offside (known as an immediate offside), while under others, it occurs only when attacking players play the puck after going offside and then failing to completely exit the offensive zone (known as a delayed offside). 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.

Under the delayed offside rule, an infraction occurs when a play is offside and any attacking player touches the puck or checks a player in the offensive zone. For example, under the National Hockey League's (NHL) delayed offside rule, play is stopped immediately when an attacking player carries the puck into the zone while a teammate is already in the attacking zone, or when an attacking player in the neutral zone completes a pass to a teammate who is already in the attacking zone. A delayed offside occurs when a player on the attacking team is in the offensive zone before the puck and the attacking team causes the puck to enter the zone when the attacking team does not have possession. When a delayed offside occurs, a linesman will keep an arm up to signal it, and all attacking players must retreat back into the neutral zone without touching the puck or checking an opponent for the delayed offside to end. If an attacking player touches the puck during the delayed offside, play is stopped.

The NHL and International Ice Hockey Federation (IIHF) apply similar rules for determining offside. A player is judged to be offside if both of their skates completely cross the blue line dividing their offensive zone from the neutral zone before the puck completely crosses the same line. In both organizations, it is the position of a player's skates that are important. They cannot use their stick or other part of their body to remain onside. The lone exception to this rule is that an attacking player's skates may precede the puck into the attacking zone when they are skating backwards if they are in control of the puck. The position of the puck is used for determining offside. Offside is determined by the skate positions when the puck completely crosses the blue line. If the puck was in the attacking zone, touches the blue line, and then completely leaves the blue line back into the attacking zone, the puck is considered to have been in the attacking zone the entire time, so there is no determination of offside, and the puck did not completely cross the blue line.

If any individual player is in an offside position, their entire team is offside. A delayed offside occurs if the puck is passed or shot into the offensive zone while an attacking player is offside, but the puck has not been touched by a member of the attacking team. In most leagues, the attacking team may "tag up" by having all players exit the offensive zone. At that point the offside is waved off and they may re-enter the offensive zone in pursuit of the puck. If a member of the attacking team has control of the puck while offside, a linesman will stop play and a faceoff will be held at the faceoff spot nearest the point of the infraction. Typically, this means the spot closest to the blue line if the puck is carried into the zone, or in the case of a pass, the spot closest to where the pass originated. If a linesman judges that the attacking team acted to force a deliberate stoppage in play by going offside, they can move the faceoff into that team's defensive zone.

If a goal is scored from a shot (at the neutral or defensive zone) that creates a delayed offside, the goal will be denied, even if the attacking team clears the offensive zone before the puck enters the goal.

Under the immediate offside rule, an infraction occurs and play is stopped as soon as a play is offside, even if the attacking team does not control the puck. Some levels of hockey use this rule, such as younger divisions of minor hockey sanctioned by USA Hockey.