What is the difference between a nacelle and a cowling?
The definition of a nacelle refers to the housing of anything on the outside of an aircraft. Engines are the most common thing in these housings.


  a streamlined housing or tank for something on the outside of an aircraft or motor vehicle.


The definition of a cowling is specifically a removable cover of the engine.


  the removable cover of a vehicle or aircraft engine.


So a nacelle refers to the whole covering of an engine that is outside the plane, typically on the wing. The cowling would just be the removable part of this cover. As Jan Hudec commented, engines mounted in the nose, as is typical in smaller aircraft, would have a cowling to allow access and cooling to the engine, but technically not a nacelle, since the covering would be part of the fuselage.