Consent to treatment is the tenet of medical ethics that requires that the patient or their medical proxy agree to a course of treatment before doctors perform a medical procedure on a patient. Where possible, that consent should be explicit and in writing.
Consent must be informed - doctors are professionally obligated to reasonably inform patients of all potential benefits and risks of a treatment. Gregory House is infamous for his browbeating of patients to get them to sign consent, and has even misled patients on several occasions.
In the absence of a medical proxy, and where the patient is unconscious, there is a presumption that a patient would consent to any procedure that a reasonable patient would agree to unless they have directed the physician not to perform such a procedure. For example, a person may make it clear that they do not want to have a transfusion for religious reasons, or may have signed a do not resuscitate order.
In some cases, such as where the patient is not mentally competent or it is suspected the medical proxy is not acting in the patient's best interests, a physician may get a court order to proceed with treatment even in the absence of a consent.