Major  —adj. 1 relatively great in size, intensity, scope, or importance. 2 (of surgery) serious. 3 mus. A (of a scale) having intervals of a semitone above its third and seventh notes. B (of an interval) greater by a semitone than a minor interval (major third). C (of a key) based on a major scale. 4 of full legal age. —n. 1  a army officer next below lieutenant-colonel. B officer in charge of a band section (drum major). 2 person of full legal age. 3 us a student's main subject or course. B student of this. —v. (foll. By in) us study or qualify in (a subject) as one's main subject. [latin, comparative of magnus great]