Messages indicate a flow of information or transition of control between elements. Messages can be used by all Interaction diagrams (except the Interaction Overview diagram) to reflect system behavior. If between Classes or classifier instances, the associated list of operations is available to specify the event.
OMG UML Specification
The OMG UML specification (UML Superstructure Specification, v2.0, p. 428) states:
"A Message defines a particular communication between Lifelines of an Interaction.
"A Message is a NamedElement that defines one specific kind of communication in an Interaction. A communication can be eg. raising a signal, invoking an Operation, creating or destroying an Instance. The Message specifies not only the kind of communication given by the dispatching ExecutionOccurrence, but also the sender and the receiver.
"A Message associates normally two EventOccurrences - one sending EventOccurrence and one receiving EventOccurrence."
Note: Communication diagrams were known as Collaboration diagrams in UML 1.4.