What is a window above a corridor doorway that allows ventilation called?

The term for a window positioned above a corridor doorway that facilitates ventilation is known as a transom. Transoms are typically used in door frames to allow light and air to enter while maintaining privacy. This design is particularly beneficial in corridors, where adequate air circulation can be crucial for comfort and safety, especially in buildings like schools, hospitals, or large commercial spaces.

By including a transom above a doorway, the building can take advantage of natural ventilation without compromising the aesthetics or structural integrity of the corridor. This is especially relevant in fire service considerations, where proper airflow can aid in smoke evacuation during emergencies.

The other terms refer to different types of windows or architectural features. A clerestory is a high section of wall that contains windows above eye level, primarily used to bring light into the interior. A casement window is one that is hinged and opens outward, commonly used for fresh air but not specifically designed for placement above doors. A skylight is a window installed in the roof or ceiling, allowing natural light to enter from above. Thus, transom is the most appropriate term for the function described in the question.