Defence (defense in the U.S.) in ice hockey is a player position whose main job is to stop the other team from scoring. They are often called defencemen, defensemen, D, or "blueliners" (a reference to the blue line in ice hockey which is boundary of the offensive zone; defencemen generally stay along the line to keep the puck in the zone).
In regular play, two defencemen play with three forwards and a goaltender on the ice. Exceptions include overtime and when a team is shorthanded (i.e. has been assessed a penalty), in which two defencemen are typically joined by only two forwards and a goaltender.