A medical ventilator (or simply ventilator in context) is a machine designed to mechanically move breathable air into and out of the lungs, to provide the mechanism of breathing for a patient who is physically unable to breathe, or breathing insufficiently.
While modern ventilators are computerized machines, patients can be ventilated with a bag valve mask, a simple hand-operated machine. After Hurricane Katrina, dedicated staff "bagged" patients in New Orleans hospitals for days with simple bag valve units attached to endotracheal tubes, a "ventilator" system which can be used with no definite time limit.
Ventilators are chiefly used in intensive care medicine, home care, and emergency medicine (as standalone units) and in anesthesia (as a component of an anesthesia machine).
Medical ventilators are sometimes colloquially called "respirators," a term which stems from commonly used devices in the 1950s (particularly the "Bird Respirator"). However, in modern hospital and medical terminology, these machines are never referred to as respirators, and use of "respirator" in this context is now a deprecated anachronism which signals technical unfamiliarity.