Pell beat me to it... but "harem" is a subclass of "romance fiction". You know.. an apple is a fruit but not all fruits are apples?
I didn't want him naming a good series that *really* wasn't a harem love romance. Even Mahoraba I term a "false" harem because
Canvas 2 at its heart is a love triangle ...
Ai Yori Aoshi is just barely a harem, really a love triangle (Aoi and Tina x Kaoru).
Shuffle! is a smack center harem anime with 5 or so girls aiming right for the hero's heart (rather than him really pursuing them).