Meaning of “major” - Learner’s Dictionary

major

noun [ C ] uk us /ˈmeɪdʒər/
SUBJECT US

the most important subject that a college or university student studies, or the student who is studying:

What's your major?
Diane's an English major.
OFFICER

an officer of middle rank in the army or air force

(Definition of “major noun” from the Cambridge Learner’s Dictionary © Cambridge University Press)