The Free On-line Dictionary of Computing (30 December 2018):

Calculus of Communicating Systems

   (CCS) A mathematical model (a formal language) for describing
   processes, mostly used in the study of parallelism.  A CCS
   program, written in behaviour expressions syntax denotes a
   process behaviour.  Programs can be compared using the notion
   of observational equivalence.

   ["A Calculus of Communicating Systems", LNCS 92, Springer
   1980].

   ["Communication and Concurrency", R. Milner, P-H 1989].

   (1994-11-29)