Definition of "Signal" :

noun: any nonverbal action or gesture that encodes a message

"Signals from the boat suddenly stopped."

noun: any incitement to action

"He awaited the signal to start."

noun: an electric quantity (voltage or current or field strength) whose modulation represents coded information about the source from which it comes

verb: be a signal for or a symptom of

"The economic indicators signal that the euro is undervalued."

verb: communicate silently and non-verbally by signals or signs

"The diner signaled the waiters to bring the menu."

adjective: notably out of the ordinary

"The year saw one signal triumph for the Labour party."