The blue line is a term used in ice hockey. A hockey rink is divided into three areas: center ice (or the "neutral zone") and one side each for the opposing teams to defend. The blue line demarcates the line between center ice and each team's zone. The blue line is important because crossing it into an opponent's zone indicates that the team is now on the attack. A hockey player is considered offside if he crosses the blue line to attack before the puck does.