An oor or odour (see spelling differences) is a volatilized chemical compound, generally at a very low concentration, which humans and other animals perceive by the sense of olfaction. Odors are also called smells, which can refer to both pleasant and unpleasant odors. The terms fragrance, scent, or 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, stench, reek, and stink are used specifically to describe unpleasant odor.