The term signal denotes a time-varying quantity that represents a medium level of information. A signal serves as a carrier of a message; It is itself carried by signs.

Signals are usually categorized in discrete and continuous spaces that the functions are defined over. This distinction applies to the corresponding time domains (e.g. discrete and continuous time domains). A second important distinction is between discrete-valued (digital) and continuous-valued signals.

In organisms, discrete-valued signals are to be found in genetically encoded information and - with regard to a single event - the all-or-none law of neuronal firing. Examples for continuous-valued signals are the firing rates of neurons and the levels of hormones, enzymes or intracellular transmitters.

In 2004, Brunner at al. showed that the velocity of signal transfer cannot exceed the speed of light in vacuum (299 792,5 km/s).