The Collaborative International Dictionary of English v.0.48:

Signal \Sig"nal\, n. [F., fr. LL. signale, fr. L. signum. See
   Sign, n.]
   1. A sign made for the purpose of giving notice to a person
      of some occurence, command, or danger; also, a sign,
      event, or watchword, which has been agreed upon as the
      occasion of concerted action.
      [1913 Webster]

            All obeyed
            The wonted signal and superior voice
            Of this great potentate.              --Milton.
      [1913 Webster]

   2. A token; an indication; a foreshadowing; a sign; anything
      taken as evidence of some process.
      [1913 Webster +PJC]

            The weary sun . . .
            Gives signal of a goodly day to-morrow. --Shak.
      [1913 Webster]

            There was not the least signal of the calamity to be
            seen.                                 --De Foc.
      [1913 Webster]

   3. Hence: (Electronics) A measureable electrical quantity,
      such as voltage or current, that conveys information by
      varying in magnitude over time; as, the signals from the
      strongest commercial radio stations can be received over
      hundreds of miles.
      [PJC] Signal to noise ratio

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]

The Collaborative International Dictionary of English v.0.48:

Signal \Sig"nal\, a. [From signal, n.: cf. F. signal['e].]
   1. Noticeable; distinguished from what is ordinary; eminent;
      remarkable; memorable; as, a signal exploit; a signal
      service; a signal act of benevolence.
      [1913 Webster]

            As signal now in low, dejected state
            As erst in highest, behold him where he lies.
                                                  --Milton.
      [1913 Webster]

   2. Of or pertaining to signals, or the use of signals in
      conveying information; as, a signal flag or officer.
      [1913 Webster]

   The signal service, a bureau of the government (in the
      United States connected with the War Department) organized
      to collect from the whole country simultaneous raports of
      local meteorological conditions, upon comparison of which
      at the central office, predictions concerning the weather
      are telegraphed to various sections, where they are made
      known by signals publicly displayed.

   Signal station, the place where a signal is displayed;
      specifically, an observation office of the signal service.
      [1913 Webster]

   Syn: Eminent; remarkable; memorable; extraordinary; notable;
        conspicuous.
        [1913 Webster]

WordNet (r) 3.0 (2006):

signal
    adj 1: notably out of the ordinary; "the year saw one signal
           triumph for the Labour party"
    n 1: any nonverbal action or gesture that encodes a message;
         "signals from the boat suddenly stopped" [syn: signal,
         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, signal, 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, signal]

The Free On-line Dictionary of Computing (19 January 2023):

SIGNAL

    A synchronous language by Le Guernic et al of
   INRIA.

   ["SIGNAL - A Data Flow-Oriented Language for Signal
   Processing," P. le Guernic, IEEE Trans Acoustics Speech &
   Signal Proc, ASSP-34(2):362-1986-04-374].

   (1996-12-10)


The Free On-line Dictionary of Computing (19 January 2023):

signal

    A predefined message sent between two
   Unix processes or from the kernel to a process.  Signals
   communicate the occurrence of unexpected external events such
   as the forced termination of a process by the user.  Each
   signal has a unique number associated with it and each process
   has a signal handler set for each signal.  Signals can be sent
   using the kill system call.

   (1996-12-10)