What are "Classics?"

From a discussion on Goodreads:

I think we might need to clarify what we mean by the term "classics." In college I took a class in philosophy class with one of the worst teachers I ever had, but I learned a number of important concepts. The one that is germane is the definition of the terms.” If you don’t agree on the definition of the term “classics” then the arguments keep going in circles.

 In another post I wrote the dictionary definition of “classic” because I was struggling with the concept even then, but it didn’t really gel until Martha mentioned a “need-to-read” list. Those two terms are not interchangeable. I think that a “need-to-read” list is something informal and it mixes books that you might call “book-group books,” and “potential classics.” I’d put the latest best sellers, books by writers like Isabel Allende, Barbara Kingsolver, William Styron, J. R. R. Tolkien, C. S. Lewis, Anne Tyler, Lee Smith, Paulo Coelho, Khaled Hosseini and many more authors, some of whom write to entertain, but some because they have something important t say. Only time will weed out the ones that have no staying power. In this group I think you will find popular books that are a waste of time to read as well as good literature.

 Classics, on the other hand, have passed the test of time. They are taught in high schools and colleges because they meld exceptional language with ideas that have a lasting impact on society. If you take a class in American Literature, you will look at the literature that our country produced on a continuum. It is like archeology of the mind. You can compare American Lit with French Lit and not only see a difference but you can see a national personality. Classics have also been reprinted again and again and in different languages. They were valuable in their day and continue to have value. You can teach a class on the meaning of the books and how the style of the author conveys his or her ideas. I think this is why some of us say that classics can’t be over-rated.