A harem is a section of a polygamous household (often of a religious nature) that's dedicated to women. It also refers to the women who occupy it. This can include the wives and concubines of the male head of household, his female relatives, and servants. In popular culture, harem has become synonymous to a group of kept women whose sole purpose is to please one man.
Contrary to the popular Western notion of what goes on within a harem, it is actually a private and sacred place for women. Men are not allowed. It was European travelers throughout history who distorted the concept of “harem” imagining it to be a sensual sanctuary where a man got his pleasure. Modern day representation of this distorted notion can be found in mass media, such as in a gaggle of women at the heels of one man.
In BDSM terms, a Harem is a group of subs serving one or more dominants.