I know, this is a book about practical game design, but when it comes to game mechanics and the core elements of gameplay, we have to go through some theory.
Formally defining anything is a difficult task; I always prefer to quote definitions from different authors. All of them are equally correct and the different points of view complement each other. It is not our goal with this book to give a comprehensive definition, what's important is that a clear idea can stick in your designer mind by reading them!
Mechanics are the various actions, behaviors, and control mechanisms afforded to the player within a game context.
–'MDA: Mechanics, Dynamics, Aesthetics Framework' by Hunicke, M. LeBlanc, R. Zubek
Game mechanics ...
