Nacelles (sometimes nacels) are supplementary exterior components on an aircraft which provide an aerodynamic enclosure for equipment which does not physically comply with the dimensions of the aircraft fuselage. Nacelles most commonly protect instruments and equipment located along an aircraft's wingspan, such as engines, fuel tanks, or weapons. In miltary aviation history, nacelles have also been used to house cockpits and other equipment on inconvential plane designs.
Nacelles are also used on horizontal axis wind turbines (HAWT). These implementations protect sensitive turbine components, such as crankshafts and gearboxes, from the environmental adversity required to power wind generation.
Images credits: Nacel Energy; Strut Patent