Olfaction is the
sense of
smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates. Many vertebrates, including most mammals and reptiles, have two distinct olfactory systems—the main olfactory system, and the accessory olfactory system. For air-breathing animals, the main olfactory system detects volatile chemicals, and the accessory olfactory system detects fluid-phase chemicals.
For water-dwelling organisms, e.g., fish or crustaceans, the chemicals are present in the surrounding water. Olfaction, along with
taste, is a form of
chemoreception. The chemicals themselves that activate the olfactory system, in general at very low concentrations, are called
odorants.