Game mechanics, in game design study (also called Ludology), are defined as "methods invoked by agents for interacting with the game world." Simply put, a mechanic is a set of rules governing the players input and the game's response in order to produce one coherent concept. For example, most shooters have an ammo mechanic governing how much a player can shoot, how long it takes to reload, and when a player can begin shooting again.
This is a companion discussion topic for the original entry at https://blog.pbbg.com/game-mechanics-introduction/