Source : Webster's Revised Unabridged Dictionary (1913)

Modal \Mo"dal\, a. [Cf. F. modal. See {Mode}.]
   1. Of or pertaining to a mode or mood; consisting in mode or
      form only; relating to form; having the form without the
      essence or reality. --Glanvill.

   2. (Logic & Metaph.) Indicating, or pertaining to, some mode
      of conceiving existence, or of expressing thought.

Source : WordNet®

modal
     adj 1: relating to or constituting the most frequent value in a
            distribution; "the modal age at which American
            novelists reach their peak is 30" [syn: {modal(a)}, {average}]
     2: of or relating to a musical mode; especially written in an
        ecclesiastical mode
     3: relating to or expressing the mood of a verb; "modal
        auxiliary"

Source : Free On-Line Dictionary of Computing

modal
     
        1. (Of an interface) Having {mode}s.  Modeless interfaces are
        generally considered to be superior because the user does not
        have to remember which mode he is in.
     
        2. See {modal logic}.
     
        3. In {MS Windows} programming, A window with the label
        "WS_MODAL" will stay on the screen and claim all the
        user-input.  Other windows can only be accessed if the MODAL
        window is closed.  Such a window would typically be used for
        an error {dialog box} to warn the user for something
        important, like "Critical error, shut down the system and
        restart".
     
        (1995-02-07)