Webster's Revised Unabridged Dictionary · 1913

Modal

Modal , adjective

[Compare French 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 & Metaphysics) Indicating, or pertaining to, some mode of conceiving existence, or of expressing thought, such as the modes of possibility or obligation.
3.
(Grammar) Pertaining to or denoting mood.

Modal , noun

(Grammar) A modal auxiliary.