Wiktionary
n. An activity to which all participants consent which is deemed to be illegal.
Wikipedia
A consensual crime is a public-order crime that involves more than one participant, all of whom give their consent as willing participants in an activity that is unlawful. Legislative bodies and interest groups sometimes rationalize the criminalization of consensual activity because they feel it offends cultural norms, or because one of the parties to the activity is considered a "victim" despite their informed consent.
Consensual crimes are sometimes described as crimes in which the victim is the state, the judicial system, or society at large and so affect the general (sometimes ideological or cultural) interests of the system, such as common sexual morality. Victimless crimes, while similar, typically involve acts that do not involve multiple persons. Drug use is typically considered a victimless crime whereas the sale of drugs between two or more persons would be a consensual crime. The fact that no persons come forward to claim injury has essentially made the two terms interchangeable in common use.