“A living will is a document that sets out a patient’s wishes regarding health care and how they want to be treated if they become seriously ill and unable to make or communicate their own choices. Living wills are also called active declarations.
Such a document may be helpful to relatives and to medical professionals in the case of a seriously ill and incapacitated patient.”

You can find an excerpt from the BBC website on Living Wills here, which gives advice and the pros and cons for making a Living Will.

Typing “Living Will” and Advance Directive for your own country into a search engine can provide you with lots of options which can relate to where you live.