An odor (American English) or odour (Commonwealth English; see spelling differences) is a smell or a scent caused by one or more volatilized chemical compounds generally found in low concentrations that humans and many animals can perceive via their olfactory system. While smell 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.