The Actor Model is a message passing concurrency model that was originally proposed by Hewitt et al. in 1973. Half a century later a plethora of variations on this model have been explored for various programming languages and systems. So much so that precise definition of actor-based programming languages is lost and that the term actor has become highly conflated. The goal of this paper is to disambiguate different actor models by classifying them into four families, namely: Classic Actors, Active Objects, Processes, and Communicating Event Loops. This paper identifies and defines the Isolated Turn Principle as the key and unifying principle among all actor models. In order to uniquely categorise the four actor families, this paper provides a precise formal definition of the core subset for each of the four families of actor models by means of an operational semantics.