This document is a written declaration instructing his or her physician to withhold or withdraw death delaying procedures in the event of a terminal condition.
Illinois law (755 ILCS 35/1) requires two witnesses.
There is some overlap between a Living Will and Heath Care Power of Attorney. A living will only applies for a terminal condition that is incurable and irreversible with imminent death. You can have both documents, but a Power of Attorney will probably cover this scenario along with broad medical authority granted to the named individual to make these decisions, including a terminal condition. A Living Will would take priority if the person in your Power of Attorney is unavailable.