living will

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

• noun A document in which the signer states his or her wishes regarding medical treatment that sustains or prolongs life, especially by invasive or extraordinary means, for use if the signer becomes mentally incompetent or unable to communicate.

from the GNU version of the Collaborative International Dictionary of English.

• A document signed by an individual expressing his/her desire that, in the event of an illness from which there is no reasonable expectation of recovery, aggressive or extraordinary measures to prolong life, such as resuscitation and life-support equipment, should not be used.

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

• noun law A document, made by a person when still legally fit to do so, expressing his desire to be allowed to die instead of being kept alive by artificial means, in the event of being severely disabled or suffering from a terminal illness.

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

• noun a document written by someone still legally capable requesting that he should be allowed to die if subsequently severely disabled or suffering terminal illness