The sense of smell or the process of detecting smells. This is achieved by receptors in olfactory organs (such as the nose) that are sensitive to air- or water-borne chemicals. Stimulation of these receptors results in the transmission of information to the brain via the olfactory nerve. See vomeronasal organ.
http://www.cf.ac.uk/biosi/staff/jacob/teaching/sensory/olfact1.html#Index Tutorial on the sense of smell compiled by Tim Jacob of Cardiff University