(Ice) hockey: One of the two 12-inch wide blue lines painted on the surface below the ice at a hockey rink. There is one 60 feet from each goal. The blue line divides the central, or neutral zone from each team's defensize zone.
The blue line is used to determine offside, which is a violation of movement rules that stops play. The blue line is also used to determine if a two-line pass was made, which also stops play.
The 'blue line' is also used informally to refer to a hockey team's on-ice defensive pairing, or sometimes the entire set of defencemen. Once could say, for example, that "Borje Salming was a stalwart member of the Toronto Maple Leafs' blue line."