Smell

The sense of smell, formally referred to as olfaction, is a sensation triggered by the reaction of receptors in the mucous membranes, of the nose that react to very small quantities of airborne chemicals. It's very closely tied together with the sense of taste. 
Several prosaic disorders that cause nasal congestion, such as the common cold or allergies, can interfere with the sense of smell. Other conditions that affect smell are rather more rare and can be caused by nerve damage or damage to the nasal tissues. More rarely, it can be caused by other unrelated conditions. There are several possible symptoms, such as an increase or decrease in olfactory perception, or having smells not correspond to smells stored in memory.
Olfaction at Wikipedia
Category:Biology