(computing) A signal that oscillates between a high and a low state and is used like a metronome to coordinate actions of digital circuits.

(rail transport) A post on which one or more railway signals are mounted.

plural of signal post

(idiomatic, informal) A thing which serves to attract or summon something.

Alternative form of cab signalling

(Britain) A time signal transmitted on radio by the BBC every hour and consisting of five short beeps followed by a longer one, with the longer one coinciding with the start of the hour.

(telephony) An audible signal on a voice phone line that sounds more frequently than a standard busy signal, indicating that no transmission path to the called phone number is available.

(US, Australia) Any of the large metal boxes containing traffic signal controls, found at roadsides and intersections, and often made available by the local council for artistic work.

plural of traffic signal box

plural of signal-to-noise ratio