The Actor model is a mathematical theory that treats "Actors" as the
universal primitives of concurrent digital computation. The model has been used
both as a framework for a theoretical understanding of concurrency, and as the
theoretical basis for several practical implementations of concurrent systems.

Unlike previous models of computation, the Actor model was inspired by
physical laws. It was also influenced by the programming languages Lisp, Simula
67 and Smalltalk-72, as well as ideas for Petri Nets, capability-based systems
and packet switching. The advent of massive concurrency through client-cloud
computing and many-core computer architectures has galvanized interest in the
Actor model.

Discretionary, adaptive concurrency is the ability to expression computations
with discretionary activity that can be adapted s to fit available resources in
terms of demand and available capacity (e.g. cores).