It is difficult to define what is classic. For example, I was born in the 20th century, so classics for me were the films from 1930s-1940s till 1980s. But for someone who is born in the 21st century, a classic film might be from 1980s to 2000s.
But, in my opinion the best definition for the word classic is to give a moral example. Such as this film does.
Justice for all, without prejudice or premonitions.

Thank you for your comment, the tips and pizza. You are very kind.