A single-player video game is a video game where input from only one player is expected throughout the course of the gaming session. "Single-player game" usually refers to a game that can only be played by one person, while "single-player mode" usually refers to a particular game mode that is designed to be played by a single-player, though the game also contains modes that can be played by several players simultaneously.
The vast majority of modern console games and arcade games are designed so that they can be played by a single-player; although many of these games have modes that allow two or more players to play (not necessarily simultaneously), very few actually require more than one player for the game to be played. The Unreal Tournament series is one example of such.
Single-player video game Wikipedia
The earliest video games, such as Tennis for Two (1958), Spacewar! (1962), and Pong (1972), were symmetrical games designed to be played by two players. Single-player games gained popularity only after this, with early titles such as Speed Race (1974) and Space Invaders (1978).
The reason for this, according to Raph Koster, is down to a combination of several factors: increasingly sophisticated computers and interfaces that enabled asymmetric gameplay, cooperative gameplay and story delivery within a gaming framework, coupled with the fact that the majority of early games players had introverted personality types (according to the Myers-Briggs personality type indicator).
Although the vast majority of modern games incorporate a single-player element either as the core or as one of several game modes, single-player gaming is currently viewed by the video game industry as peripheral to the future of gaming, with Electronic Arts president Frank Gibeau stating that he had not approved one game to be developed as a single-player experience,.
As the narrative and conflict in single-player gameplay is created by a computer rather than a human opponent, single-player games are able to deliver certain gaming experiences that are typically absent - or de-emphasised - in multiplayer games.
Single-player games rely more heavily on compelling stories to draw the player into the experience and to create a sense of investment. Humans are unpredictable, so human players - allies or enemies - cannot be relied upon to carry a narrative in a particular direction, and so multiplayer games tend not to focus heavily on a linear narrative. By contrast, many single-player games are built around a compelling story.
Whilst a multiplayer game relies upon human-human interaction for its conflict, and often for its sense of camaraderie, a single-player game must build these things artificially. As such, single-player games require deeper characterisation of their non-player characters in order to create connections between the player and the sympathetic characters and to develop deeper antipathy towards the game's antagonist(s). This is typically true of role-playing games (RPGs), such as Dragon Quest and the Final Fantasy series, which are primarily character-nagato.
It should be noted that these game elements are not firm, fixed rules; single-player puzzle games such as Tetris or racing games focus squarely on gameplay.