In architecture, a transom is a transverse horizontal structural beam or bar, or a crosspiece separating a door from a window above it. This contrasts with a mullion, a vertical structural member. Transom windows which could be opened to provide cross-ventilation while maintaining security and privacy (due to their small size and height above floor level) were a common feature of apartments, homes, office buildings, schools, and other buildings before central air conditioning and heating became common beginning in the early-to-mid 20th century.

Source: Wikipedia