Source : Webster's Revised Unabridged Dictionary (1913)

Signal \Sig"nal\, v. t. [imp. & p. p. {Signaled or Signalled};
   p. pr. & vb. n. {Signaling} or {Signalling}.]
   1. To communicate by signals; as, to signal orders.

   2. To notify by a signals; to make a signal or signals to;
      as, to signal a fleet to anchor. --M. Arnold.

Source : WordNet®

signal
     adj : notably out of the ordinary; "the year saw one signal
           triumph for the Labour party"
     [also: {signalling}, {signalled}]

signal
     n 1: any communication that encodes a message; "signals from the
          boat suddenly stopped" [syn: {signaling}, {sign}]
     2: any incitement to action; "he awaited the signal to start";
        "the victory was a signal for wild celebration"
     3: an electric quantity (voltage or current or field strength)
        whose modulation represents coded information about the
        source from which it comes
     [also: {signalling}, {signalled}]

signal
     v 1: communicate silently and non-verbally by signals or signs;
          "He signed his disapproval with a dismissive hand
          gesture"; "The diner signaled the waiters to bring the
          menu" [syn: {sign}, {signalize}, {signalise}]
     2: be a signal for or a symptom of; "These symptoms indicate a
        serious illness"; "Her behavior points to a severe
        neurosis"; "The economic indicators signal that the euro
        is undervalued" [syn: {bespeak}, {betoken}, {indicate}, {point}]
     [also: {signalling}, {signalled}]

signalling
     See {signal}