A Signal is a specification of Send request instances communicated between objects, typically in a Class or Package diagram. The receiving object handles the Received request instances as specified by its receptions. The data carried by a Send request is represented as attributes of the Signal. A Signal is defined independently of the classifiers handling the signal occurrence.
OMG UML Specification
The OMG UML specification (UML Superstructure Specification, v2.1.1, pp. 447-448) states:
"A signal triggers a reaction in the receiver in an asynchronous way and without a reply. The sender of a signal will not block waiting for a reply but continue execution immediately. By declaring a reception associated to a given signal, a classifier specifies that its instances will be able to receive that signal, or a subtype thereof, and will respond to it with the designated behavior."