An actor is anything in a game that is capable of deciding to do something in the game (taking an action) in order to try and achieve a win. In board and card games, actors are generally players. In videogames they can be either players (through dolls), characters or opponents.
Actors are not simple rule systems that react to action, nor objects that follow a pattern behaviour. The deck of cards in Solitaire is not an actor, for example, nor is a floating platform in a Mario game.
Actors in this sense should not be confused with dramatic actors from film or theatre. They should also not be confused with voice actors. While some narrativists may conflate these meanings (as they often do) as a way to prove that games are stories, what is meant by the use of the word ‘actor’ in game is simply a class identification of entities that do things, trigger conditional rules and cause state changes. Nothing more.