Mass. Gen. Laws ch. 201D § 1
As used in this chapter the following words shall, unless the context clearly requires otherwise, have the following meanings:
"Attending physician", the physician, selected by or assigned to a patient, who has primary responsibility for the treatment and care of the patient, in whatever setting medical diagnosis or treatment is rendered. Where more than one physician shares such responsibility, any such physician may act as the attending physician.
"Capacity to make health care decisions", the ability to understand and appreciate the nature and consequences of health care decisions, including the benefits and risks of and alternatives to any proposed health care, and to reach an informed decision.
"Facility", any facility as defined in section seventy E of chapter one hundred and eleven.
"Health care", any treatment, service or procedure to diagnose or treat the physical or mental condition of a patient.
"Health care agent" or "agent", an adult to whom authority to make health care decisions is delegated under a health care proxy.
"Health care decision made by an agent under a health care proxy", a decision which is made in accordance with the requirements of this chapter, is consistent with any limitations in the health care proxy, and is consistent with responsible medical practice.
"Health care provider", an individual or facility licensed, certified, or otherwise authorized or permitted by law to administer health care in the ordinary course of business or professional practice.
"Health care proxy", a document delegating to an agent the authority to make health care decisions, executed in accordance with the requirements of this chapter.
"Principal", a person who has executed a health care proxy.
Mass. Gen. Laws ch. 201D, § 1