Signaling is the act of presenting a change of state in the hopes of sharing information across some distrance. Signaling in human biological terms includes gestures, facial expressions, and speech.
Within the networking world, signaling can be considered the act of informing each hop along a network's path of how to recognize traffic for which a special processing behavior is required and indicating the type of processing that must be carried out. Signaling must also take into consideration the constraints of the network, to define limits on the requested resources by means of authentication.
See also: Semaphore, Signaling System, RSVP, & Differentiated Services.