Persistent vegetative state (PVS) is the name, coined by physicians and now widely used, of a specific condition of drastically impaired consciousness. That condition is characterized as ‘vegetative’ on the basis of the behaviour of patients, not of their underlying neurological disorder or disorders (Jennett 1993: 40). This behaviour includes the following features: The patient has periods of wakefulness and sleep, in contrast to patients in coma who are in a sleep–like condition. During the periods of wakefulness the patient exhibits reflex actions and can swallow small amounts of food and water by mouth, but does not exhibit voluntary action or psychologically meaningful interaction with the environment. Patients in a vegetative state breathe spontaneously (Institute of Medical Ethics Working Party 1991: 96–97).

Vegetative state can be transient, but it can persist. This persistence becomes medically and morally significant when the vegetative state continues long enough to justify an expectation that it will be permanent, that is, if the vegetative state continues to the point that there is no medical prospect that treatment will have any effect on the patient's severely impaired consciousness. The medical judgment that the vegetative state of any given patient is permanent is based on prospective trials and clinical observation; it depends on such considerations as the age and condition of the patient and the cause of the neurological disorder, not on direct observation of this disorder (Institute of Medical Ethics Working Party 1991: 96–97; Jennett 1993: 40–41).