A stimulus is a communication between instances that conveys information with the expectation that action will ensue. A stimulus will cause an operation to be invoked, raise a signal, or cause an instance to be created or destroyed.

Different types of stimuli can be used to model different flows of control:

Type	Shape	Description
Procedure Call or Nested Flow of Control		Models either a call to an operation or a call to a nested flow of control. When calling a nested flow of control, the system waits for the nested flow of control to complete before continuing with the outer flow.
Asynchronous Flow of Control		Models an asynchronous stimulus between two objects. The source object sends the stimulus and immediately continues with the nest step.
Return from a Procedure Call		Models a return from a call to a procedure. This type of stimulus can be omitted from diagrams because it is assumed that every call has a return.