A ventilator is a machine that brings in good air that's safe to breathe and keeps bad air out. A kitchen ventilator uses fans and filters to direct greasy stove exhaust outside.
There are architectural ventilators, for keeping the air inside buildings clean, and also medical ventilators, which work to keep breathable air moving in and out of a patient's lungs. This kind of ventilator is only used when someone is unable to breathe effectively on their own. In both cases, ventilators are all about air. The word comes from the Latin root ventulus, "a breeze."