DEVS is a formal, modular framework for specifying discrete-event simulation models, developed by Bernard Zeigler in the 1970s. It defines two building blocks: an atomic model, which has its own state, internal and external transition functions, an output function, and a time-advance function; and a coupled model, which wires atomic models together into hierarchies through explicit port connections.
DEVS underpins much of modern Discrete-Event Simulation (DES) theory. It is expressive enough to encode most other discrete-event formalisms, and its modular structure makes models composable and closed under coupling, meaning that any valid coupling of DEVS models is itself a DEVS model. Extensions such as Hybrid DEVS allow continuous dynamics to be combined with discrete events, bridging to System Dynamics-style modelling.