I think of modern as something that, when it was new, was notable enough for just being new. Then, when the newness has passed away, people were used to calling whatever that was ‘modern’, but now it’s attached to something no longer new.
One of my first encounters with this was a class called “Modern English Novels”, which ran from around 1900 (Kipling’s “Kim”) to the 20s (Ulysses and To The Lighthouse).
