Anosmia (pronounced /ænˈɒzmiə/) is a lack of functioning olfaction, or in other words, an inability to perceive odors. Anosmia may be temporary but traumatic anosmia can be permanent. Anosmia is due to an inflammation of the nasal mucosa; blockage of nasal passages or a destruction of one temporal lobe. It can be caused by chronic meningitis and neurosyphilis that would increase intracranial pressure over a long period of time, and in some cases by ciliopathy including ciliopathy due to primary ciliary dyskinesia (Kartagener syndrome, Afzelius’ syndrome or Siewert’s syndrome). Many patients may experience unilateral anosmia, often as a result of minor head trauma. This type of anosmia is normally only detected if both of the nostrils are tested separately. Using this method of testing each nostril separately will often show a reduced or even completely absent sense of smell in either one nostril or both, something which is often not revealed if both nostrils are simultaneously tested.
A related term, hyposmia, refers to a decreased ability to smell, while hyperosmia refers to an increased ability to smell. Some people may be anosmic for one particular odor. This is known as “specific anosmia”. The absence of the sense of smell from birth is called Congenital Anosmia.
(Note: The information above is courtesy of Wikipedia)