Defence or defense (in American English) in ice hockey is a player position that is primarily responsible for preventing the opposing team from scoring. They are often referred to as defencemen, D, D-men or blueliners (the latter a reference to the blue line in ice hockey which represents the boundary of the offensive zone; defencemen generally position themselves along the line to keep the puck in the zone). They were once called cover-point.
In regular play, two defencemen complement three forwards and a goaltender on the ice. Exceptions include overtime during the regular season and when a team is short-handed (i.e. has been assessed a penalty), in which two defencemen are typically joined by only two forwards and a goaltender; when a team is on the power play (i.e. the opponent has been assessed a penalty), teams will often play only one defencemen, joined by four forwards and a goaltender. In National Hockey League regular season play in overtime, effective with the 2015-16 season, teams (usually) have only three position players and a goaltender on the ice, and may use either two forwards and one defenceman, or—rarely—two defencemen and one forward.