ETYM From signal, n: cf. French signalé.
Notably out of the ordinary.

1. To notify by a signal
2. To communicate or indicate by or as if by signals; to constitute a characteristic feature of (a meaningful linguistic form)
3. To make or send a signal

Any sign, gesture, sound, or action that conveys information.
Examples include the use of flags (semaphore), light (traffic and railroad signals), radio telephony, radio telegraphy (Morse code), and electricity (telecommunications and computer networks).
The International Code of Signals used by shipping was drawn up by an international committee and published 1931. The codes and abbreviations used by aircraft are dealt with by the International Civil Aviation Organization, established 1944.
An electrical transmittance (either input or output) that conveys information.

1. Any electrical quantity, such as voltage, current, or frequency, that can be used to transmit information.
2. A beep or tone from a computer’s speaker or a prompt displayed on screen that tells a user that the computer is ready to receive input.