This action was to delineate the propfan engine type from a number of ducted engine proposals at the time that had propfan in their names.[4] By the 2000s, open rotor (OR) became a preferred term for propfan technology in research and news reports, with contra-rotating open rotor (CROR) also occasionally being used to distinguish between single-rotation propfans. 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." The engine uses a gas turbine to drive an unshrouded (open) contra-rotating propeller like a turboprop, but the design of the propeller itself is more tightly coupled to the turbine design, and the two are certified as a single unit.[5]