/ˈsɪgnəl/ noun [C]
ACTION › a movement, light, or sound which 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 › a series of electrical waves which are sent to a radio, television, or mobile phone
PROOF › something which 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 which 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