major in (something)
To complete a college or university degree with a particular subject as its focus. Originally I thought I would major in English literature, but I switch to psychology in my second year. Samantha's majoring in economics, with a minor in history.
major in something
to specialize in a certain subject in college. I majored in history in college. I want to major in math.
To complete a long-term course of study in some subject that is one's main focus: In high school I was interested in math, but in college I majored in history.