You use major when you want to describe something that is more important, serious, or significant than other things in a group or situation.
A major is an officer who is one rank above captain in the British army or the United States army, air force, or marines.
At a university or college in the United States, a student's major is the main subject that they are studying.
At a university or college in the United States, if a student is, for example, a geology major, geology is the main subject they are studying.
If a student at a university or college in the United States majors in a particular subject, that subject is the main one they study.
In music, a major scale is one in which the third note is two tones higher than the first.
A major is a large or important company.
The majors are groups of professional sports teams that compete against each other, especially in American baseball.
A major is an important sporting competition, especially in golf or tennis.