An undergraduate is a college or university student who's not a graduate student. After high school, you can become an undergraduate.
Undergraduates are students of universities and colleges: they've graduated from high school and have been accepted to college, but they haven't graduated yet. If the under in undergraduate seems confusing, that's because there are also graduate students, students who have a college degree but are no longer undergraduates and are pursuing an advanced degree, like a Master's or Ph.D. Lots of people say being an undergraduate was one of the best times of their lives: it's an exciting thing to be.
