When a game played does not involve another human, is usually considered a single-player. It is the opposite of multi-player. There are games that do not allow for any other kind of play (such as Tetris or Dungeons) while other games do not even include a single-player (early on Counter-Strike was such a game, as it did not feature any bots in the beginning).
Single-player does not automatically mean the same as „single-player campaign“. Playing only against bots in multiplayer settings can also be considered single-player but is not referred to as a campaign.
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