Dysosmia, also known as olfactory dysfunction, is the impairment of olfactory stimuli processing leading to an altered sense of smell.
These dysfunctions can present in a variety of ways, such as the stimuli not activating the olfactory bulb, some odors being interpreted as other odors, or hallucinations of smells. These subsets are called anosmia, parosmia, and phantosmia respectively. Anosmia and hyposmia (a less severe form of anosmia with a decreased sensitivity to smell but not the complete lack of it) are more quantitative disabilities typified by degrees in which the odor is sensed. Parosmia and phantosmia are more qualitative disabilities that are denoted by the impromptu introduction of odors without the proper stimulus.
(Note: The information above is courtesy of Wikipedia)