"Classic" books

What would you consider a classic? Surely it can't just be a book that was written in the 19th century or earlier?

There was probably a lot of stuff published back then that wouldn't be considered a classic. And there are probably books being written even today that fit the bill?

What are the criteria?

And which are your favourites?

I am at present reading "Kidnapped" by Robert Louis Stevenson as I had an old Book club podcast on my phone for yonks and I thought I should read the book before I listened to it

Before that I read the Prime of Miss Jean Brodie and then listened to a podcast with Muriel Spark doing a Q and A. Strange connection: Sandy in the Brodie story has a crush on Alan Breck in Kidnapped! (Who wouldn't?)

Both are books that can surely be classified as classic literature.