As of 2015, the European Aviation Safety Agency (EASA) defined an open rotor concretely (but broadly) as ""a turbine engine fan stage that is not enclosed within a casing;"" in contrast, it had only a working definition of an open rotor engine (the more commonly used term for propfan in the 21st century), calling it ""a turbine engine featuring contra-rotating fan stages not enclosed within a casing.""