A genre of anime or manga where the main character is surrounded by multiple characters of the opposite sex. Can often be a yaoi or yuri. A standard harem involves one male character and multiple female characters. If the main character is female it is referred to as a reverse harem.

(n.) 1630s, from Turkish harem, from Arabic haram "wives and concubines," originally "women's quarters," literally "something forbidden or kept safe," from root of harama "he guarded, forbade."

To quote someone who knows what they're talking about:
"From Arabic harIm (literally "something forbidden") and haram (literally "a sanctuary")"

Although its original meaning was the part of a household reserved for females - including perhaps a man's mother, sisters, and daughters, as well as wife or wives - it is commonly used to mean a group of more than one woman who are all available sexually to a single man. This situation is Polygyny ("multiple women.") Sometimes it also refers to a group of men who are available to a single woman. This situation is Polyandry ("multiple men.") Either may be called Polygamy ("multiple marriage.")

People who think it derives from Latin don't know what they're talking about, which is no great crime - no one knows everything. People who think it derives from Latin and say so on a public forum like this not only don't know that they don't know what they're talking about, but are displaying and perpetuating their ignorance.

Like every mans dream. Of having a shit load of women everywhere.

What you say when you see a hot girl/guy who you'd "put in your harem" - implies superficial attraction but not wanting to go for long term.

Obviously a reference to the Arabic حرم 'ḥaram' 'forbidden place; sacrosanct, sanctum': a place where female members of a polygamous wealthy family would reside. And be fucked at the husband's leisure. Similar to a 'seraglio'.

A group of females of three or more who are loose moral whores and only like dick and MAC beauty products. They walk together in groups and go into stores like Juicy Couture and PINK by Victoria Secret. They can be caught wearing tight yoga pants (even though they don't do yoga), lacey bras with sheer lacey shirts, too much makeup, Jeggings, and regular tans. If you see one of these, don't be afraid to call out "Harem". They can be seen on multiple campuses across the nation of the United States.

(n.) A woman's sacred place, that which a man cannot enter; a nun's vagina; a back in the day term for a stable full o' bitches.