major (IMPORTANT)
adjective [before noun]
more important, bigger or more serious than others of the same type:
All of her major plays have been translated into English.
Sugar is a major cause of tooth decay.
There are two problems with this situation, one major, one minor.
Citrus fruits are a major source of vitamin C.
There has been a major change in attitudes recently.
The United States is a major influence in the United Nations.
Compare minor (UNIMPORTANT).

majorly
adverb MAINLY US SLANG
very or extremely:
Have you seen Chrissie's new leather jacket? It's majorly cool.

major (SPECIAL SUBJECT)
noun [C] US
the most important subject that a college or university student is studying, or the student himself or herself:
What is your major, English or French?
She was a philosophy major at an Ivy League college.

major in sth verb US
to study something as your main subject at university:
She majored in philosophy at Harvard.