A classic film, to me, is one that achieves an artistic vision (demonstrating concern with theme and story, but critically focusing on a means of conveying a theme) and also entertains, captivates, and demands re-viewing.
Some classics that come to mind: Network, The Conversation, Persona, Failsafe, The Empire Strikes Back...
As with books, there are many definitions of classic. Some films and books are described as instant classics, which seems to be a contradiction in terms. Therefore a classic is not just a movie that can be watched and enjoyed by generations, and holds meaning. A classic is also a movie that we see now that we predict will have those properties.
Classic films, like classical music, art, and literature, are replete with meaning, significant dialogue, techniques, and superb actors. There are many different levels at which these films can be viewed.
One movie that brought three actors Academy Awards and was a great satire is the 1975 film Network. But, what few, if any, viewers knew at the time was how prophetic this movie was. Now, in 2010 the movie hardly seems a satire; at any rate it is not exaggerated. This ability of a film to transcend a time period seems also a measure of "a classic."
Whenever students enjoy an old film and comment that it is "really good," someone invariably beats me to my remark: "It's a classic!"
I would agree with mshurn's assessment. It's rather like literature isn't it? Classic films will endure, and they will inspire, and they will keep people coming back to watch them. Interestinig thing to consider--how many "classic" films are based on "classic" literature? For example: Cat on a Hot Tin Roof.
I read somewhere that a "classic" anything was at least 100 years old, but the term clearly has a wider application. The commonality, though, is that a classic withstands the test of time; it lasts because of its enduring appeal, generation after generation. In terms of film, a classic must combine excellence in many respects of cinema, but something else is involved, I think.
A film classic is more than an excellent movie; it is an excellent film that explores universal truths in a unique way. The "unique" elements of a classic film might include photography and scenic design (Orson Wells' Citizen Kane), or a new way to present an old genre, such as the incredibly suspenseful movies of Alfred Hitchcock. Much of Hitchcock's success in creating suspense also involved new and creative uses of the camera. Sometimes a film classic is a movie that breaks new cultural ground by telling a story previously untold in film--or by presenting a compelling vision of the future.