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 defenceman, 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.

Organized play of ice hockey originates from the first indoor game in Montreal in 1875. In subsequent years, the number of players per side was reduced to seven. Positions were standardized, and two correspond to the two defencemen of current six-man rules. These were designated as cover point and point, although they lined up behind the center and the rover, unlike today. Decades later, defencemen were standardized into playing left and right sides of the ice.

According to one of the earliest known books on ice hockey, Farrell's Hockey: Canada's Royal Winter Game (1899), Mike Grant of the Montreal Victorias, describes the point as "essentially defensive. He should not stray too far from his place, because oftentimes he is practically a second goal-minder ... although he should remain close to his goal-keeper, he should never obstruct that man's view of the puck. He should, as a rule, avoid rushing up the ice, but if he has a good opening for such a play he should give the puck to one of the forwards on the first opportunity and then hasten back to his position, which has been occupied, in the interim, by the cover-point."

Also in Farrell's 1899 book, Hugh Baird, then captain and cover-point of the Montreal Hockey Club, describes the cover-point as "a combination of a defence man and a forward, and is allowed, in virtue of this fact, more latitude with respect to leaving his position, than any man on the team, except the rover. In his capacity of a defence player, he should linger around his goals as long as the puck is near... When the play is at the other end of the rink, the cover-point should advance to about the middle, so that when the puck is lifted down, he may return it without loss of time, in order to keep the game centered around his opponents' goals, and to save his forwards the trouble of skating up to him so that they may again 'get in to play.' It is by playing far up under these circumstances that a clever cover-point can chine to the advantage of his team. If he has a good opening he should shoot well for the goals, but if he has not, he should, as I have said, return the puck instantaneously."

After a finished season the NHL, the premier ice hockey league in the world, presents the James Norris Memorial Trophy to the best defenceman in the league. Bobby Orr of the Boston Bruins – an eight-time Norris Trophy recipient – is often considered to be the greatest defenceman in NHL and ice hockey history. In addition to his Norris Trophy honours, he is the only defenceman in NHL history to capture the Art Ross Trophy as the league's leading scorer. In 1998, Orr was selected as the best defenceman of all-time (second overall player behind Wayne Gretzky) in The Hockey News' Top 100 NHL Players of all-time.

Conversely, according to the IIHF Centennial All-Star Team (also chosen by The Hockey News), the greatest defencemen to play in IIHF-sanctioned international competition are Vyacheslav Fetisov and Börje Salming.

Defence players are often described by the amount that they participate in offence. The extreme of non-participation in offence is a "stay-at-home" defender, who plays a conservative, risk averse game and does not score much, instead focusing on defensive assignments towards the opposing team. A good example is Rod Langway, who won the 1983 Norris Trophy after having scored only three goals that season, as the award winners before and after him were primarily offensive defencemen such as Bobby Orr, Denis Potvin, and Paul Coffey. Other examples of stay-at-home defencemen include Leo Boivin, Chris Chelios, Tim Horton, Harry Howell, Tom Johnson, Jacques Laperrière, Kevin Lowe, Brad McCrimmon, Bill Quackenbush, and Scott Stevens.