A
signal as referred to in
communication systems,
signal processing, and
electrical engineering is a function that "conveys
information about the behavior or attributes of some phenomenon". In the physical world, any quantity exhibiting variation in time or variation in space (such as an image) is potentially a signal that might provide information on the status of a physical system, or convey a
message between observers, among other possibilities. The
IEEE Transactions on Signal Processing states that the term "signal" includes
audio,
video, speech,
image, communication, geophysical, sonar, radar, medical and musical signals.