Longman Dictionary of Contemporary English - major

~1 adj 1 usually before noun very large or important, when compared to other things or people of a similar kind  (There are two major political parties in the US. | Mahler's music was a major influence on the young composer. | one of the major causes of cancer)  (- opposite minor1 (1)) 2 usually before noun having very serious or worrying results  (The loss of Cantona through injury was a major setback for the team. | major problem)  (This could create major traffic problems.) 3 AmE spoken very important  (This is major? You got me out of bed for this?) 4 a major key (key1 (4)) is based on a musical scale in which there are semitones between the third and fourth and the seventh and eighth notes  (a symphony in D major)  (- compare minor1 (2)) ~2 n 1 an officer of middle rank in the British or US army or marines, or in the US airforce  (- see also drum major) 2 especially AmE the main subject that a student studies at college or university  (Her major is history.)  (- compare minor2 (2)) 3 AmE someone studying a particular subject as their main subject at college or university  (She's a history major.) 4 the majors the Major Leagues  (- compare minor league) ~3 v major in sth phr v especially AmE to study something as your main subject at college or university  (He's majoring in Political Science.)