1. The condition of supplying fresh air and getting rid of foul air: Ventilation of the bedroom was necessary before the children went to bed that night.
2. A mechanical system in a building that provides fresh air: The old system of ventilation needed to be replaced with a new one in the ancient dwelling.
3. The bodily process of inhalation and exhalation: Ventilation includes the process of taking in oxygen from inhaled air and releasing carbon dioxide by exhalation.
4. The replacement of stale or noxious air with fresh air: Ventilation is a process of moving or circulating air so as to supply outside air to an enclosed space and/or to remove stale air from the area with the purpose of cooling, purification, moisture reduction, etc., which may or may not involve mechanical conditioning.

Mechanical ventilation used in assisting breathing when deficient: Assisted-controlled ventilation can also completely assume control of breathing in the presence of apnea (non breathing).

Mechanical ventilation that takes the place of natural inspiration and expiration: Controlled ventilation can be employed in the management of pulmonary insufficiency.

A system employing an exhaust fan that forces out foul or used air from a room in a building and provides fresh air from outside: Some firms, where dangerous or noxious processes take place, use vacuum ventilation in order to provide a healthy environment for the workers.

Methane released to the atmosphere from the ventilation systems of underground coal mines: Because methane is a greenhouse gas, technologies have been developed to degasify the ventilation air methane by means of oxidation.