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Gameplay is arguably the most important part of a game. It is, simply, how the game plays. The controls, the fun you derive from playing it, what you do in the game, the interactivity - these are all aspects of gameplay.
Outside of computer games, gameplay refers to what the player does (such as how the player can manipulate pieces.)
Within computer games, it will also include AI behavior, whether the AI simply follows a basic movement pattern, is a normal player in the game, or is a cheating AI that uses its own ruleset.