Signal

A signal is both the process and result of transmission of data over some media. It is accomplished by embedding some variation.

Definitions specific to sub-fields are common: - In applied mathematics, a signal is a function mapping from the time domain to some codomain which can be anything that is measurable. - In electronics and telecommunications, signal refers to any time-varying voltage, current, or electromagnetic wave that carries information. - In signal processing, signals are analog and digital representations of analog physical quantities. - In information theory, a signal is a codified message, that is, the sequence of states in a communication channel that encodes a message. - In a communication system, a transmitter encodes a message to create a signal, which is carried to a receiver by the communication channel. For example, the words "Mary had a little lamb" might be the message spoken into a telephone. The telephone transmitter converts the sounds into an electrical signal. The signal is transmitted to the receiving telephone by wires; at the receiver it is reconverted into sounds. - In telephone networks, signaling, for example common-channel signaling, refers to phone number and other digital control information rather than the actual voice signal.

There are digital and analog signals. Source: https://en.wikipedia.org/wiki/Signal