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 Unified Modeling Language specification, (v2.5.1, p.178) states:
A Signal is a specification of a kind of communication between objects in which a reaction is asynchronously triggered in the receiver without a reply.
The OMG Unified Modeling Language specification, (v2.5.1, p.178) also states:
A Reception is a declaration stating that a Classifier is prepared to react to the receipt of a Signal.