PhD student is also correct. When I was in school, PhD student meant you were enrolled in the PhD program, but PhD candidate usually meant you were writing your thesis or going through defense. "Student" is less formal and more conversational in any case.

Good question! I never thought about it, but while we say graduate student, we do say PhD candidate. I don't know why, it sounds more mature, I suppose. :)

Thanks for your comment, Shaggy!

A Ph.D. candidate is the one who passed their candidacy exam (i.e. defended their proposal) while a Ph.D. student is the one who has recently entered the program. Normally, after two years, a Ph.D. student will be converted to a Ph.D. candidate.