The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. (Logic & Metaph.) Indicating, or pertaining to, some mode
      of conceiving existence, or of expressing thought, such as
      the modes of possibility or obligation.
      [1913 Webster +PJC]

   3. (Gram.) Pertaining to or denoting mood.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Modal \Mo"dal\(Gram.), n.
   A modal auxiliary.
   [PJC]

WordNet (r) 3.0 (2006):

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"
    n 1: an auxiliary verb (such as `can' or `will') that is used to
         express modality [syn: modal auxiliary verb, modal
         auxiliary, modal verb, modal]

The Free On-line Dictionary of Computing (30 December 2018):

modal

   1. (Of an interface) Having modes.  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)