A nymphomaniac is a woman with an excessive sex drive.
The problem is that it’s hard to define “excessive.” Wanting to have sex every week? Every day? Every hour? The real issue is not frequency, but control. Someone’s sex drive can become a problem if the person has no control over it (see: sex addiction).
It’s an old fashioned term that’s hardly ever used by sexologists nowadays, because it has no established meaning and has a negative connotation. In the nineteenth century it was used to force women along established lines, making them feel sick or abnormal if they had any sexual desire or were sexually active. If a woman for instance wanted to make love more often than her male companion she could be accused of being a nymphomaniac.
Today it’s considered healthy if a girl desires sex, turning the term “nympho” into a teasing compliment.
Guys sometimes have the idea it’s exciting to date a girl who wants sex all the time. But in reality it may work out badly if he is unable to satisfy her. It can have a negative effect on his self confidence and the sexual relationship may become a burden. It’s much easier to develop a good sexual relationship if lovers’ sex drives match.