Harem

Harem, broadly speaking, is a loose subgenre of anime and manga characterized by a protagonist surrounded, usually amorously, by three or more members of the opposite sex. The most common and practically tantamount scenario is a male surrounded by a group of females; when this is conversed it is informally referred to as a reverse harem. The term is derived from the Arabic "harem", modernly defined outside its original meaning as "a group of women associated in any way with one man or household"

Structure
Daryl Surat, a contributor to Otaku USA, suggests that many harems are built around one primary fetish while the female cast individually satisfy others

Because romance is rarely the main focus of an entire series, harem structure is ambiguous. The most distinguishable trait is arguably the group of girls who accompany, and in some instances cohabitate with the boy, and while intimacy is just about customary, it is never necessary; when it is present, there must be a minimum of three girls who express it, otherwise two is a love triangle. Additionally, it is not essential for there to be one exclusive boy; many can exist as long as they are given less attention or the story calls for an unusually obscure sex ratio.