In tabletop games and video games, game mechanics define how a game works for players. Game mechanics are the rules or ludemes that govern and guide player actions, as well as the game's response to them. A rule is an instruction on how to play, while a ludeme is an element of play, such as the L-shaped move of the knight in chess. The interplay of various mechanics determines the game's complexity and how the players interact with the game. All games use game mechanics; however, different theories disagree about their degree of importance to a game. The process and study of game design includes efforts to develop game mechanics that engage players.

Common examples of game mechanics include turn-taking, movement of tokens, set collection, bidding, capture, and spell slots.

There is no consensus on the precise definition of game mechanics. Competing definitions claim that game mechanics are:

A game's mechanics are not its theme. Some games have a theme—some element of representation. For example, in Monopoly, the events of the game represent another activity, the buying and selling of properties. Two games that are mechanically similar can be thematically different, and visa versa. The tension between a game's mechanics and theme is ludonarrative dissonance.

Abstract games do not have themes, because the action is not intended to represent anything. Go is an example of an abstract game.

Some game studies scholars distinguish between game mechanics and gameplay. In Playability and Player Experience Research, the authors define gameplay as "the interactive gaming process of the player with the game." In this definition, gameplay occurs when players interact with the game mechanics. Similarly, in Dissecting Play – Investigating the Cognitive and Emotional Motivations and Affects of Computer Gameplay, the authors define gameplay as "interacting with a game design in the performance of cognitive tasks". Video games researcher Carlo Fabricatore defines gameplay as:

In Ernest Adams and Andrew Rollings on game design, the authors define gameplay as the combination and interaction of many elements of a game.

However, popular usage sometimes elides the two terms. For example, gamedesigning.org defines gameplay as the core game mechanics that determine a game's overall characteristics.