The sense of smell or the process of detecting smells. This is achieved by receptors in olfactory organs (such as the nose in vertebrates) 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. Fishes have olfactory receptors located in pits on the snout just in front of the eyes. Each pit is lined by a folded sensory epithelium, which detects chemicals in water flowing through the pit, via incurrent and excurrent openings (nares). Chemicals in the water are also detected by taste buds located over the body surface. Insects smell airborne odorants using olfactory hairs on their antennae. See vomeronasal organ.

https://www.cardiff.ac.uk/people/view/81198-jacob-tim Tutorial on the sense of smell compiled by Tim Jacob of Cardiff University