(sciences) A figure of merit comparing the strength of a signal carrying information to the noise interfering with it.

plural of signal-to-noise ratio

plural of fast busy 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.

(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