Wiktionary
n. A propeller which is housed inside a duct to improve engine performance of aircraft.
Wikipedia
A ducted fan is a propulsion arrangement whereby a mechanical fan, which is a type of propeller, is mounted within a cylindrical shroud or duct. The duct reduces losses in thrust from the tips of the props, and varying the cross-section of the duct allows the designer to advantageously affect the velocity and pressure of the airflow according to Bernoulli's Principle. Ducted fan propulsion is used in aircraft, airships, airboats, hovercraft and fan packs.
Ducted fans normally have more and shorter blades than propellers and thus can operate at higher rotational speeds.
Usage examples of "ducted fan".
The swing wings are strictly for a supersonic dash at low altitude, using ducted fan and perhaps small auxiliary jets buried in the aft hull, drawing air from the fan plenum.
Margo waited until they were well out of sight of the little bayside community below, then fired up the ducted fan engines.
Sitting on its spidery legs the skimmer was a spindly, open-frame box built around a ducted fan, with a skirt of flexible metal mesh draped around its base.
Wheels, chains, propeller, spikes, ducted fan, or a team of oxen if that's yer karma.