An odor is caused by one or more volatilized chemical compounds,
generally at a very low concentration, that humans or other
animals perceive by the sense of olfaction. Odors are also
commonly called scents, which can refer to both pleasant and
unpleasant odors. The terms fragrance, scent, and aroma are used
primarily by the food and cosmetic industry to describe a
pleasant odor, and are sometimes used to refer to perfumes. In
contrast, malodor, stench, reek, and stink are used specifically
to describe unpleasant odors. The sense of smell gives rise to
the perception of odors, mediated by the olfactory nerve. The
olfactory receptor (OR) cells are neurons present in the
olfactory epithelium, a small patch of tissue in back of the
nasal cavity. Odor sensation depends on the concentration
(number of molecules) available to the olfactory receptors. A
single odorant stimulus type is typically recognized by multiple
receptors, and different odorants are recognized by combinations
of receptors, the patterns of neuro