The 200405 NHL lockout was a labor lockout that resulted in the cancellation of the National Hockey League (NHL) season, which would have been its 88th season of play. The main dispute was the league's desire to implement a salary cap to limit expenditure on player salaries. This was opposed by the NHL Players Association (NHLPA), the players' labor union, who proposed an alternative system of revenue sharing.

Attempts at collective bargaining before the season began were unsuccessful. The lockout was initiated on September 16, 2004, one day after the expiration of the existing collective bargaining agreement (CBA), which itself had been the result of the 199495 lockout. During the lockout, further attempts to negotiate a new CBA floundered, with neither side willing to back down, leading to the entire season being canceled in February 2005. The NHL and NHLPA negotiating teams finally reached an agreement on July 13, 2005, with the lockout officially ending 9 days later on July 22, after ratification by the NHL team owners and NHLPA members. The resulting CBA included both a salary cap and revenue sharing.

The lockout had lasted 10 months and 6 days, covering 1,230 unplayed games. As a result, the Stanley Cup was not awarded, for the first time since 1919. Among the major professional sports leagues in North America, this was the first (and so far only) time a whole season was canceled because of a labor dispute, and the second time a postseason was canceled (after the 199495 MLB strike). Large numbers of NHL players elected to play in European leagues during the lockout.

The National Hockey League (NHL; French: Ligue nationale de hockey—LNH, French pronunciation: [liɡ nasjɔnal də ɔkɛ]) is a professional ice hockey league in North America comprising 32 teams—25 in the United States and 7 in Canada. It is considered to be the premier professional ice hockey league in the world, and is one of the major professional sports leagues in the United States and Canada. The Stanley Cup, the oldest professional sports trophy in North America, is awarded annually to the league playoff champion at the end of each season. The NHL is the fifth-wealthiest professional sport league in the world by revenue, after the National Football League (NFL), Major League Baseball (MLB), the National Basketball Association (NBA), and the English Premier League (EPL).The National Hockey League was organized at the Windsor Hotel in Montreal on November 26, 1917, after the suspension of operations of its predecessor organization, the National Hockey Association (NHA), which had been founded in 1909 in Renfrew, Ontario. The NHL immediately took the NHA's place as one of the leagues that contested for the Stanley Cup in an annual interleague competition before a series of league mergers and foldings left the NHL as the only league left competing for the Stanley Cup in 1926.

At its inception, the NHL had four teams, all in Canada, thus the adjective "National" in the league's name. The league expanded to the United States in 1924, when the Boston Bruins joined, and has since consisted of both American and Canadian teams. From 1942 to 1967, the league had only six teams, collectively (if not contemporaneously) nicknamed the "Original Six". The NHL added six new teams to double its size at the 1967 NHL expansion. The league then increased to 18 teams by 1974 and 21 teams in 1979. Between 1991 and 2000, the NHL further expanded to 30 teams. It added its 31st and 32nd teams in 2017 and 2021, respectively.

The league's headquarters have been in Midtown Manhattan since 1989, when the head office moved from Montreal. There have been four league-wide work stoppages in NHL history, all occurring after 1992. The International Ice Hockey Federation (IIHF) considers the Stanley Cup to be one of the "most important championships available to the sport". The NHL draws many highly skilled players from all over the world and currently has players from approximately 20 countries. Canadians have historically constituted the majority of the players in the league, with an increasing percentage of American and European players in recent seasons.