Hmmm. You are making me really think this over even more, Angie! I don't think I put my thumb on it the first go.
I think of a classic as timeless and iconic to some degree. To me, classic means shapes that form part of our style vocabulary. Turtle/polo necks, crew necks, v-necks, scoop necks, boat necks. Wrap dress, sheath dress, fit and flare dress. A-line skirt, pencil skirt, etc. Classics wax and wain because they don't always dominate the style scene, but you can usually find "current" adaptations of them to let you continue to wear them. For example, a few years ago when a-lines were "the" shape, it was harder to find pencil skirts. However, you could find them, but with a slightly more flared shape than the classic. That adaptation let them skim by and look current. It converted the classic into a modern classic adaptation.
And all this is entirely only reflecting my on disjointed thinking and theories--I claim absolutely no credibility in making this distinction!!!