plural of Greenwich Time Signal

(rail transport, British spelling) An electric railway signal with colour aspects, showing either red or green in a 2-aspect signal, with the addition of a yellow or double yellow aspect (meaning caution) in 3- and 4-aspect signals.

plural of colour light signal

(rail transport, UK) A signal given to a train driver by the guard of the train, or a member of station staff, that the train is ready to depart.

(electronics, technology) The use of digital processing, such as by computers or more specialized digital signal processors, to perform a wide variety of signal processing operations.

(networking, telecommunications) A type of transmission technology in which signaling is separate from the data being transmitted, using one or more channels for transmitting data or voice information and one out-of-band channel for performing signaling functions.