From Wikipedia:
Informed consent is a legal condition whereby a person can be said to have given consent based upon an appreciation and understanding of the facts and implications of an action. The individual needs to be in possession of relevant facts and also of his or her reasoning faculties, such as not being mentally retarded or mentally ill and without an impairment of judgment at the time of consenting. Such impairments might include illness, intoxication, insufficient sleep, and other health problems.
Inform Your Consent 