Modal

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 ]

fld>(Gram.), n. Any one of the auxiliary verbs of English, such as can, may, will, shall, must, might, could, would, or should, which are used together with the infinitive form of another verb to express distinctions of mood{ 2 }, such as uncertainty, possibility, command, emphasis, and obligation. [ PJC ]

n. (Theol.) One who regards Father, Son, and Spirit as modes of being, and not as persons, thus denying personal distinction in the Trinity. Eadie. [ 1913 Webster ]

n. [ Cf. F. modalité. ] 1. The quality or state of being modal. [ 1913 Webster ]

2. (Logic & Metaph.) A modal relation or quality; a mode or point of view under which an object presents itself to the mind. According to Kant, the quality of propositions, as assertory, problematical, or apodeictic. [ 1913 Webster ]

n. A system of logic which studies how to combine propositions which include the concepts of necessity, possibility, and obligation. [ PJC ]