What makes a novel "a classic"?

I think what makes a novel a classic is how old it is and how popular it was during it's time. If someone were to mention a book and you had no idea what it was then it probably wasn't a classic. Usually classics are well-renowned, even maybe in a different country. Classics are usually known by older people, because they come from before their time, and maybe as I grow I will be exposed to more classics.