You're probably confusing it with loli, which is pedo stuff.

I don't think there's a strict dictionary definition because it's a slang term, but imo Yuri means the work focuses on romantic relationships between women (or girls). Utena is definitely yuri. Haven't seen Shin Sekai Yori but if there are at least two female leads and they're at least semi-plausibly gay, it's also yuri.