Calling someone a nymphomaniac or accusing them of nymphomania isn't something that can be defined by science.
Nymphomania is a layperson's term used to label a woman, or a nympho, whose sex drive or sexual activity is subjectively deemed too high.
The term "nymphomania," is not scientifically meaningful simply because there are no specific criteria that would define a nymphomaniac. In other words, there isn't a way to determine how much sexual desire or activity is too much.
The clinical conditions that include the concept of high levels of sexual desire and/or activity are hypersexuality and sexual addiction or compulsivity.
The central features of these disorders are that sexual activity is an insatiable need, often interfering with other areas of everyday functioning; sex is impersonal, with no emotional intimacy; and despite frequent orgasms, sexual activity is generally not satisfying.
The label of nymphomania is used in a pejorative and derogatory manner, almost exclusively in reference to women. To many men, the idea of a woman with a greater sex drive than their own is somewhat threatening, so they may use the label to preserve their own egos by "proving" that the woman is abnormal.
Similarly, men with sexual dysfunction might accuse their partners of being oversexed in an effort to hide their own fears or sense of inadequacy, just as some women who object to the frequency of their partner's sexual advances might accuse him of being oversexed.
The difference is that the double standard which exists in our society congratulates a man who is highly sexed and has many partners, calling him a "stud," whereas a woman with the same behavior is often called a "nympho," which carries a negative connotation.