Day of Defeat is a team-based multiplayer first-person shooter video game set in the European theatre of World War II on the Western front. Originally a modification of the 1998 game Half-Life, the rights of the modification were purchased by Valve Corporation and released as a full retail title in 2003. The game was generally well-received, with an aggregate score of 79/100 on Metacritic.

Set in the midst of World War II, Day of Defeat includes no single-player campaign, with focus left only on the game's multiplayer aspects. The game favors teamwork and features objective-based gameplay in combination with its system of classes. Maps are primarily made up of narrow paths, all of which typically lead to a few key locations. An official remake of the game, Day of Defeat: Source, was released by Valve in 2005. Utilizing the then-new Source engine, the remake featured various improvements over its predecessor, including updates to gameplay, graphics, and audio functionality.