Stimulus transmitted between organisms.
To be a signal the stimulus must both originate from one organism and be received by another. Note, though, that the origin of the signal need not be intentional on the part of the originator nor necessarily even be "" or otherwise accurate.
The recipient, in turn, may or may not act upon receiving a signal. If action occurs and it is a fixed action pattern, however, then the signal can be described as a kind of sign stimulus.
Communication between organisms requires the origination of a signal by one organism and its reception by another. Note that this communication, however, need not be two way, that is, the recipient need not signal in return for communication still to have taken place.