A model for concurrency by Carl Hewitt. Actors are autonomous and concurrent objects which execute asynchronously. The Actor model provides flexible mechanisms for building parallel and distributed software systems.
["Laws for Communicating Parallel Processes", C. Hewitt et al, IFIP 77, pp. 987-992, N-H 1977].
["ACTORS: A Model of Concurrent Computation in Distributed Systems", Gul A. Agha <firstname.lastname@example.org>, Cambridge Press, MA, 1986].