What is the term for the restroom on an aircraft?

The term used for the restroom on an aircraft is "lavatory." This word specifically refers to the compact and efficient restroom facilities designed for use in airplanes, which often include a toilet and basic washing facilities. The term is widely recognized in the aviation industry and is standard terminology used by flight crews and in-flight announcements.

Other terms like "rest cabin" and "washroom" are more general and can refer to restroom facilities in various contexts, not specifically on an aircraft. “Flight toilets” could also convey the necessary meaning but is less commonly used compared to "lavatory." Thus, "lavatory" is the correct and most appropriate term for referring to aircraft restrooms.