Smell. An odor, or odour, is caused by one or more volatilized chemical compounds that are generally found in low concentrations that humans and animals can perceive by their sense of smell.
   An odor is also called a smell or a scent, which can refer to either a pleasant or an unpleasant odor. While scent can refer to pleasant and unpleasant odors, the terms scent, aroma, and fragrance are usually reserved for pleasant-smelling odors and are frequently used in the food and cosmetic industry to describe floral scents or to refer to perfumes.
   In the United Kingdom and other Commonweath, English-speaking nations, odour refers to scents in general-without positive or negative connotations; but in the United States, and for many non-native English speakers around the world, odor generally has a negative connotation as a synonym for stink. An unpleasant odor can also be described as reeking or called a malodor, stench, pong, or stink.
   The perception of odors, or sense of smell, is mediated by the olfactory nerve. The olfactory receptor cells are neurons present in the olfactory epithelium, which is a small patch of tissue at the back of the nasal cavity.
   There are millions of olfactory receptor neurons that act as sensory signaling cells. Each neuron has cilia in direct contact with the air. Odorous molecules bind to receptor proteins extending from cilia and act as a chemical stimulus, init

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