A document that sets out guidelines for dealing with life-sustaining medical procedures in the eventuality of the signatory's sudden debilitation.
Living wills would, for example, inform medical staff not to provide extraordinary life-preserving procedures on their bodies if they are at that time unable or incapable of expressing themselves and suffering from an incurable and terminal condition.
For the most part, a misnomer, as a will does not by definition take effect until the signatory's death whereas a "living will" is useless after the signatory's death.