Literally, a student is one who studies. But in Scientology terms, it means more than that. It really means someone who is studying Scientology technology using Hubbard's methodology of study. There is a brief introduction to this Study tech in the article, Barriers to study.
The word tends to also mean a person enrolled on a formal course in a formal courseroom, rather than a person applying study tech on his own at home or at a university somewhere. Of course an undergraduate is also a student, at least when he is studying, but not in the sense the word is used in a Scientology environment. It is on a par with pc, the student being one learning to become an auditor, and the pc being one receiving auditing.
The full technology of study is given in a course called the Student Hat. This course includes the full theory of study, and eight or so taped lectures on the subject. The core tapes sometimes have others added and sometimes not, depending on which checksheet is being used. This course is usually a pre-requisite to any major training course, such as an Academy Level or the Solo Auditor Course.