What Constitutes a Classic in Crime Fiction?

Someone said on a post last week that perhaps Elmore Leonard was not really a classic mystery writer. Is it Golden Age writers that wear that hat?

How would you define "classic?" Is it an age thing? Is it a style thing? Is it the number of books sold thing? Is it a longevity thing? Is it " in the eye of the beholder"  thing?

What makes a book a classic? (To me Elmore Leonard would qualify.)