Definitions of signal:

• noun:   any communication that encodes a message

  Example: "Signals from the boat suddenly stopped"

• noun:   any incitement to action

  Example: "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:   communicate silently and non-verbally by signals or signs

  Example: "The diner signaled the waiters to bring the menu"

• verb:   be a signal for or a symptom of

  Example: "The economic indicators signal that the euro is undervalued"

• adjective:   notably out of the ordinary

  Example: "The year saw one signal triumph for the Labour party"