Meaning of “signal” - Learner’s Dictionary

signal

noun [ C ] uk us /ˈsɪɡnəl/
ACTION

B2 a movement, light, or sound that gives information, or tells people what to do:

At a signal from their teacher, the children all held up their flags.
Don't move until I give the signal.
WAVE

B2 a series of electrical waves that are sent to a radio, television, or mobile phone

PROOF

something that shows that something else exists or is likely to happen:

The changing colour of the leaves on the trees is a signal that it will soon be autumn.
TRAINS

a piece of equipment that tells trains to stop or to continue

VEHICLES/PEOPLE US

a piece of equipment that shows people or vehicles when to stop, go, or move carefully:

a traffic signal

(Definition of “signal noun” from the Cambridge Learner’s Dictionary © Cambridge University Press)