The problem with the term "classic literature" is that everyone has a different definition. For example, I consider Joe Haldeman's Forever War to be a good example of classic literature, but most English instructors would call it sophmoric sci-fi, assuming they'd deign to read something so far below their stature.

What classic literature is, is determined by popular consensus- in other words, it's a popularity contest.

Classic literature could also be defined as "something and English major forces you to read long before you're ready to read it for yourself.