Offside (ice hockey)
In ice hockey, a play is offside if a player on the attacking team enters the attacking zone before the puck does, unless the puck is sent or carried there by a defending player. If an attacking player is attempting to shoot the puck into the attacking zone and it merely deflects off a defending player before entering the zone, an offside violation can still occur. This is unlike the icing rule, where an unintentional deflection by the other team will negate an icing call.
When an offside violation occurs, the linesman blows the play dead, and a faceoff is conducted in the neutral zone closest to where the infraction occurred.
Elements that determine offside 
An offside violation has two elements, both of which must be satisfied for offsides to be established:
- Puck position: the puck must completely cross the edge of the blue line closest to the attacking zone, thus entering the attacking zone.
- Attacking players' skate position: when the puck enters the attacking zone, the skates of at least one attacking player are already fully within the attacking zone.
These rules mean that a player can be in an offside position within the attacking zone when the puck is outside of the zone; an offside call is made only when the puck enters the attacking zone.
An attacking player's skates may precede the puck into the attacking zone provided he is in control of the puck and no other offside condition exists when the puck completely crosses the determining edge of the blue line.
Immediate vs. delayed vs. tag-up offside 
Though the basic offside rule is always the same, there are different rules for the situation when the puck is shot in while a player is offside, but the defending team gains control of the puck.
- With immediate offside, play is dead the instant an offside violation occurs. Immediate offside was used in the USA Hockey youth leagues until 2011.
- Delayed offside allows the defending team a chance to move the puck out of the zone. The play remains offside until the puck enters the neutral zone. Play is blown dead if the defending team does not attempt to move the puck forward. Simple delayed offside was used in the major North American leagues in the AHL until 2004 (except from 1986-96), in the ECHL until 2005 (except from 1996–2005), and in the NHL until 2005 (except from 1986–96).
- Tag-up offside is a variation of delayed offside. In a delayed offside situation, if all members of the offending team clear the defensive zone at the same time by making skate contact outside of the zone (including the blue line), then the delayed offside call is negated. Tag-up offside is used in NCAA, Hockey Canada, the IIHF, USA Hockey junior and youth leagues, some North American professional and adult leagues, and the NHL from 1986-1996, and once again after the 2005 Collective Bargaining Agreement.
Play is stopped immediately if a player from the attacking team touches the puck in the attacking zone while he or any of his teammates is offside.
Offside during faceoffs 
Offside is also used to refer to a player lining up on his opponent's side during a faceoff. Failure of a team to properly line up for the faceoff may result in the ejection of the offending team's center; If the violation persists, a delay of game penalty may be assessed.
Offside pass 
An offside pass or second (blue) line pass is a pass from inside a team's defending zone that crosses the red line. When such a pass occurs, play is stopped and a faceoff is conducted in the defending zone of the team that committed the infraction.
There are two determining factors in an offside pass violation:
- Puck position when pass is released. Since the blue line is considered part of the zone the puck is in, if the puck is behind or in contact with the blue line when the pass is released, the pass may be an offside pass.
- Skate position of the receiver. If the receiver has skate contact with the red line at the instant the puck completely crosses it, the pass is legal regardless of where the puck actually makes contact with his stick. Both of his skates must be completely on the far side of the red line when the puck crosses the red line into the attacking zone is governed by the aforementioned offside rule.
This offside pass rule is not observed by all leagues. For instance, it was abolished by the IIHF, and its member countries' leagues (except the NHL) in 1998. The National Hockey League recently adopted the version used by the top minor leagues, under the terms of their 2005 Collective Bargaining Agreement, in which the center line is no longer used to determine a two-line pass. This was one of a number of rule changes intended to open up the game and improve scoring chances, making the game more exciting for the fans.