major

n. military rank above captain and below colonel; adult, one who has reached legal majority; main subject of study chosen by a college or university student; major key (Music); major scale (Music)

v. specialize in a certain field of study

adj. more important than others; great; serious; having reached legal majority; pertaining to a main subject of study chosen by a college or university student; of a major scale (Music); based on a major scale (Music)

Major is a military rank the use of which varies according to country. Moreover, Major frequently denotes a mid-level command status officer (immediately superior to the rank of Captain and immediately subordinate to the rank of Lieutenant Colonel), however in some armies major is essentially a senior NCO (non-commissioned officer), a usage derived from "sergeant-major".
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n. major, Maj., military rank above captain and below colonel

n. major, military rank above captain and below colonel

adj. Canis Major, southern constellation