1.an opening or a device that allows fresh air to come into a closed space
The bathroom had no window but it had an electric ventilator.
2.a machine that helps people breathe when they have difficulty breathing on their own, by forcing air in and out of their lungs
He was brought into intensive care shortly after the accident and immediately put on a ventilator.
Three days after surgery, she was taken off the ventilator, but had to go back on it a day later.