signal >> ˈsɪɡn(ə)l
Definition: [noun] a gesture, action, or sound that is used to convey information or instructions, typically by prearrangement between the parties concerned.
Example: the firing of the gun was the signal for a chain of beacons to be lit
Definition: [noun] an electrical impulse or radio wave transmitted or received.
Example: equipment for receiving TV signals
Definition: [noun] 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.
Example: the driver of the mail train overran a signal at danger and ran into the rear of an express
Definition: [verb] convey information or instructions by means of a gesture, action, or sound.
Example: hold your fire until I signal