Phantosmia, or olfactory hallucinations, involves smelling odors that are not derived from any physical stimulus. These phantom odors can range from rotting flesh to a spring meadow, though most cases report unpleasant aromas. In some cases phantosmia has even led to the afflicted individual to believe that both the odor and source in fact exist, a condition known as olfactory delusions.
Phantosmia on Wikipedia.
