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

Petri net

    A directed, bipartite graph in
   which nodes are either "places" (represented by circles) or
   "transitions" (represented by rectangles), invented by Carl
   Adam Petri.  A Petri net is marked by placing "tokens" on
   places.  When all the places with arcs to a transition (its
   input places) have a token, the transition "fires", removing a
   token from each input place and adding a token to each place
   pointed to by the transition (its output places).

   Petri nets are used to model concurrent systems,
   particularly network protocols.

   Variants on the basic idea include the coloured Petri Net,
   Time Petri Net, Timed Petri Net, Stochastic Petri Net,
   and Predicate Transition Net.

   FAQ (http://daimi.aau.dk/PetriNets/faq/answers.htm).

   (1996-09-10)