There aren't many, but there are some being made these days that can be watched and appreciated in 50 years. Or more.

It's extremely subjective, but that is the nature of art.

(Dude, that's classic!, e.g.) that the term is meaningless to me, a guy in his mid-fifties.

A good movie is one that tells a story well, and that features good actors believable in their roles. A great one is one I'll watch several times in appreciation of the craft demonstrated by the collective creative effort involved.

But yes--I agree it's a subjective call. No arguing taste, as they say.

I think it's one that helps start or define an era (artistic or historical) or a body/collection of work (by an actor/director/studio/etc).

The older a film gets the more likely it falls into one of those categories. But then I'll look at a film like Up in the Air (one of my favorites of the past five years) and readily label it an instant classic. It's a great story that brilliantly defined a period of American history and was a showcase for the talents of George Clooney, director Jason Reitman and newcomer Anna Kendrick.