Definition

A passenger service unit (PSU) is a module installed above each passenger seat that contains reading lights, a flight attendant call button, a seat belt sign, a no smoking sign, gaspers (vents), and oxygen masks that deploy in the event of cabin depressurization. Features may vary somewhat depending on aircraft model. 

Virtually all transport-category passenger airplanes carry this equipment, but some smaller commuter aircraft may not be fitted with PSUs.