Real Dictionary

signal in American English
a token or indication
adjective: serving as a signal; used in signaling
intransitive verb: to make communication by a signal or signals
noun: anything that serves to indicate, warn, direct, command, or the like, as a light, a gesture, an act, etc.
transitive verb: to make a signal to

signal in British English
adjective: distinguished or conspicuous
noun: any sign, gesture, token, etc, that serves to communicate information
verb: to communicate (a message, etc) to (a person)

signal in Electrical Engineering
noun: A signal is a series of radio waves or changes in electrical current that may carry information.