A legal document that gives authority to a trusted person to make any and all decisions if someone is unable to do so.

With respect to end-of-life planning, this document is typically part of an Advance Directive, a series of legal documents that state the type of medical care a person would like to receive, or who they may want to speak on their behalf, if they lack the capacity to do so during a medical emergency.

In this case, it's also known as Medical Power of Attorney.