[1] : The Collaborative International Dictionary of English v.0.48

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.
      1913 webster

   2. to notify by a signals; to make a signal or signals to;
      as, to signal a fleet to anchor. --m. arnold.
      1913 webster
see also:
signaled or signalled signaling signalling 

[2] : WordNet (r) 2.0

signal
     adj : notably out of the ordinary; "the year saw one signal
           triumph for the labour party"
     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
     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
see also:
signaling sign signalize signalise bespeak betoken 
indicate point signalling signalled 

[3] : WordNet (r) 2.0

signalling
     see signal
see also:
signal 

Signalling >> S Words
Signalling, definition of term: Signalling
signalling_pag1.html

Signalling Syst >> S Words
Signalling Syst, definition of term: Signalling Syst
signalling+syst_pag1.html