Greater than others in importance or rank: a major artist. Great in scope or effect: a major improvement. Great in number, size, or extent: the major portion of the population. Requiring great attention or concern; very serious: a major illness. Law Having attained full legal age. Of or relating to the field of academic study in which a student specializes. Music Designating a scale or mode having half steps between the third and fourth and the seventh and eighth degrees. Music Equivalent to the distance between the tonic note and the second or third or sixth or seventh degrees of a major scale or mode: a major interval. Music Based on a major scale: a major key. A commissioned rank in the U.S. Army, Air Force, or Marine Corps that is above captain and below lieutenant colonel. One who holds this rank. One that is superior in rank, importance, or ability: an oil-producing country considered as one of the majors. Law One who has reached full legal age. A field of study chosen as an academic specialty. A student specializing in such studies: a linguistics major. Logic A major premise. Logic A major term. Music A major scale, key, interval, or mode. Music A chord containing a major third between the first and second notes and a minor third between the second and third notes. Sports The major leagues. To pursue academic studies in a major: majoring in mathematics.