Quantitative and qualitative dysfunctions
| Term | Definition |
| Hyposmia | Reduced perception of smell, can be specific to one odorant or to all odours |
| Anosmia | Complete absence of perception of smell; usually trigeminal inputs remain, and some chemically irritating odours may still be perceived |
| Functional anosmia | Olfactory function is present, but is too poor to be useful in daily life |
| Parosmia | Distorted perception of odour in the presence of an odorous stimulus |
| Phantosmia | Perception of odour without an external stimulus |