DEVS (Discrete EVent Systems specification) was created by Bernard Zeigler  . It is a modular formalism for deterministic and causal systems. It allows for component-based design of complex systems. A DEVS model may contain two kinds of DEVS components: Atomic DEVS and Coupled DEVS. An Atomic DEVS does not contain any component in it. It only has a mathematical specification of its behavior. A Coupled DEVS is a modular composition of one or more Atomic DEVS'.
According to the closure under coupling property of DEVS, a Coupled DEVS can be substituted by an Atomic DEVS with equivalent behavior. A Coupled DEVS can be used to compose more complex DEVS components.
A Coupled DEVS specifies connections between the components in it. Two connected components send messages via well-defined ports.