minor in (something)
To complete a college or university degree with a particular subject as an official secondary focus. Originally I thought I would major in English literature, but I ended up minoring in it. Samantha majored in economics and minored in history.
minor in something
to study a secondary subject in college. (Compare this with major in something.) I minored in math in college. I decided to minor in history.
To complete a long-term course of study in some subject that is one's secondary focus: My main course of study is biology, and I'm minoring in philosophy.