A formal, graphical, executable technique for the specification and analysis of concurrent, discrete-event dynamic systems; a technique undergoing standardisation. More on Petri nets...
The technique belongs to a branch of mathematics called graph theory. A Petri net may be represented graphically as well as mathematically. The ability to visualise structure and behaviour of a Petri net promotes understanding of the modelled system.
Software tools exist which support graphical construction and visualisation.