An action is something that a character can do during a role-playing game. Although some games may use the term action to mean something specific within their own systems (see below), when used as a general term it is anything that the characters can do or perform when they get the opportunity. (The timing of a character's action may depend on the initiative system of the game.)
Specific uses in games
As well as being a general term for anything a character can do, some games use the term action to mean something specific in their systems. This may be strictly mechanical, or may affect the game world.
For example, in Fate Core characters can do anything that makes sense in the fiction, but whatever they do is translated into the rules of the game as one of four actions: attack, defend, overcome, and create an advantage. That is, the characters perform actions in the general sense, but the term is defined in the game rules to refer to these four specific game mechanics.
In games like Dungeons & Dragons 5th edition, however, the term action is applied only to a finite and fixed subset of things a character can do. When characters do something not on the list of actions, they might instead be performing a bonus action, a reaction, a free action, or a movement (for example). In this way, not only is action given a mechanical definition, but this definition is incompatible with the general use of the term used on this wiki.