Anime "harems" are usually shows were a guy is surrounded by
women, many/most/all of whom want him (romantically and/or
sexually), although it can be a woman/girl who's the center of
affection (Fushigi Yuugi and Maze being examples of this). At a
minimum, there have to be three people interested in the main
character; it can't just be a love triangle. Four or more
people chasing after the main character almost certainly makes it
a harem situation, while just three might or might not, depending
on the biases of the person doing the judging (for instance, I
don't consider Yawara to fall under this category).
Series that I consider to fall under this category are:
Some characteristics of a harem series are:
-
The center of affection is never lecherous, and never
takes advantage of the situation. Ranma might seem to be an
exception to this, but he only likes the situation because
having all the beautiful women after him is a big boost to his
ego; he couldn't make himself kiss the gorgeous Shampoo for a
(supposed) cure to his Jusenkyo curse. And while there might
be four people after the female Maze, when she turns into the
lecherous male Maze, there's only one person interested in him.
-
Who the center of affection ends up with is usually not resolved
until the end of the series, if ever; the only exceptions are
Fushigi Yugi and El Hazard. The center of the love tangle
either has no romantic interest in any of them (Tenchi), seems
to like them all the same (Otaru of SMJ), constantly waffles
(Keitaro of Love Hina), has a preference but is too
shy/nice/interrupted to do anything about it (Tasuke of Mamotte
Shugogetten!), or has a preference but won't admit it, even to
himself (Ranma).
-
Should the main character become the object of a tug-of-war or a
turf battle, he/she will tolerate it, or at most run away,
rather than telling those doing the fighting to grow up.