# What is an Actor?

A [Syndicated Actor][Actor] is a collection of stateful
[Entities][Entity], organised in a tree of [Facets][Facet], with each
facet representing a
[(sub)conversation](https://syndicate-lang.org/about/#conversational-concurrency-1)
that the Actor is engaged in. Each entity belongs to exactly one
facet; each facet has exactly one parent and zero or more children;
each actor has exactly one associated root facet. When a facet is its
actor's root facet, its parent is the actor itself; otherwise, its
parent is always another facet.

In the taxonomy of De Koster *et al.* ([2016](#DeKoster2016)), the
Syndicated Actor model is a *Communicating Event-Loop* actor model,
similar to that offered by the E programming language
([Wikipedia](https://en.wikipedia.org/wiki/E_(programming_language));
[erights.org](http://erights.org/)).

 - [Actor], [ActorRef], [Facet], [FacetRef], [ActorState], [Mailbox],
   [Activation]

**References.**

 - De Koster, Joeri, Tom Van Cutsem, and Wolfgang De Meuter. <a
   name="DeKoster2016">“43 Years of Actors: A Taxonomy of Actor Models
   and Their Key Properties.”</a> In Proc. AGERE, 31–40. Amsterdam,
   The Netherlands, 2016.
   [DOI](https://doi.org/10.1145/3001886.3001890).
   [PDF](http://soft.vub.ac.be/Publications/2016/vub-soft-tr-16-11.pdf).