The actor *(paradigm) is a *(term) for modelling computation with _(concurrent) (possibly transparently-_(distributed)) entities, the actors, that communicate with asynchronous messages. Actors may update their behaviour depending on the messages they receive.

See the pioneering works by Carl Hewitt, the developments by his successor Gul Agha, or by other fine people like Akinori Yonezawa. Actors have been rediscovered twenty years later by formal-methods people, yielding the _(join calculus). They have also been rediscovered by the industry, and a nice language that robustly implements the actor model is _(Erlang).

Actors are the real thing of which _(object-oriented) programming is the caricature. When people say that OOP is based on an "intuitive" modelling of the world as objects in interaction, the intuition they invoke is the Actor model, but the actual model is a less-expressive version where only one object is active at a time, with a linear flow of control.