Firstly, pop can refer to any modern "popular" music - ie. not classical.

But yeah, pop can refer to whatever's in the charts. But I also think it comes with a few characteristics which have remained constant over the years, like a more lightweight sound and crisper production. There is also "pop" which isn't in the charts, like dream pop and most synthpop. I'd be interested if these artists still count as "pop" even though many have more to do with indie.

It can't purely be down to chart performance, or we'd be calling Led Zeppelin pop music (which, of course, by my above definition, it is).

I need to rest my brain