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.
A Reception is defined as a feature of the receiving object, derived from the Signal element. The Reception takes the name of the Signal, and the Signal's attributes as its parameters.
OMG UML Specification:
The OMG UML specification (UML Superstructure Specification, v2.1.1, p.450) 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.
And (UML Superstructure Specification, v2.1.1, p.447 - 448):
A reception is a declaration stating that a classifier is prepared to react to the receipt of a signal. A reception designates a signal and specifies the expected behavioral response. The details of handling a signal are specified by the behavior associated with the reception or the classifier itself. ... Receptions are shown using the same notation as for operations with the keyword <signal>