A lewd act is a legal category that describes a crime that is of a sexual nature yet falls short of the legal definition of sexual assault or rape. The U.S. Military defines unlawful sexual contact as unwanted contact with the victim's genitals or surrounding areas with the intent to arouse or degrade any person involved.
A lewd act is illegal, but the punishment for it varies from one jurisdiction to another as well as from one case to another. A case involving lewdness against minors, for instance, is frequently punished to a greater extent than a similar offense against an adult. The State of Idaho, for example, assigns harsh penalties to anyone convicted of lewd acts with a child younger than 16. These acts, which are delineated in detail on the state's legislative website, may be punished with a term in state prison for a term that it describes as "not less than life."
Even some activities that are not strictly intended as sex acts can be covered by the blanket definition of lewdness. Wikipedia offers the examples of streaking, mooning and skinny dipping, which are all illegal activities because they fall under the subject of lewd conduct laws.