Frotteurism refers to a sexual diorder wherein obtainment of sexual gratification is by rubbing one's genitals against or fondling the body parts of a nonconsenting person
Other /More definition:
Frotteurism refers to Paraphilia in which the person gains sexual gratification by rubbing against unwilling victims in crowds from which they cannot escape.
Moreover, Frotteurism is paraphilia in which the individual has intense sexual urges and sexually arousing fantasies of rubbing against or fondling an unsuspecting stranger.