a gesture, action, or sound that is used to convey information or instructions, typically by prearrangement between the parties concerned

the firing of the gun was the signal for a chain of beacons to be lit

the policeman raised his hand as a signal to stop

an electrical impulse or radio wave transmitted or received

equipment for receiving TV signals

an apparatus on a railway, typically a coloured light or a semaphore, giving indications to train drivers of whether or not the line is clear

the driver of the mail train overran a signal at danger and ran into the rear of an express

convey information or instructions by means of a gesture, action, or sound

hold your fire until I signal

striking in extent, seriousness, or importance; outstanding

their signal failure to achieve a satisfactory solution to the problem