An outrunner is a type of brushless electric motor.
This type of motor spins its outer shell around its windings, much like motors found in ordinary CD-ROM computer drives. In fact, CD-ROM motors are frequently rewound into brushless outrunner motors for small park flyer aircraft. Parts to aid in converting CD-ROM motors to aircraft use are commercially available.
Outrunners spin much slower than inrunner motors (though still considerably faster than ferrite motors) while producing far more torque. This makes an outrunner an excellent choice for directly driving electric aircraft propellers since they eliminate the extra weight and complexity of a gearbox.
See also: Inrunner