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

an electrical impulse or radio wave transmitted or received

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

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

striking in extent, seriousness, or importance; outstanding