of, relating to, or characteristic of mode or manner
(grammar) (of a verb form or auxiliary verb) expressing a distinction of mood, such as that between possibility and actuality. The modal auxiliaries in English include can, could, may, must, need, ought, shall, should, will, and would
qualifying or expressing a qualification of the truth of some statement, for example, as necessary or contingent
relating to analogous qualifications such as that of rules as obligatory or permissive
(metaphysics) of or relating to the form of a thing as opposed to its attributes, substance, etc
1560s, term in logic, from Middle French modal and directly from Medieval Latin modalis "of or pertaining to a mode," from Latin modus "measure, manner, mode" (see mode (n.1)). Musical sense is from 1590s.