victimless crime

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An illegal act that results in no identifiable person being directly harmed.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An act or behavior which is prohibited by law, yet which neither directly harms nor violates the rights of any specific person, although some people may claim it harms society as a whole. This concept typically applies to adults.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an act that is legally a crime but that seem to have no victims