From Academic Kids
An odor (American English) or odour (Commonwealth English) is the object of perception of the sense of olfaction. Odors are also called smells. The term stench is used to describe an unpleasant odor. The term fragrance or aroma is used primarily by the food and cosmetic industry to describe a pleasant odor.
Odors correspond to the objective phenomenon of chemicals dissolved in air, although, as with other senses, psychological factors can play a part in perception.
Certain odors, such as perfumes and flowers, are much sought after and large prices are paid for the most elite ones. Other whole industries have developed products to remove unpleasant odors. See deodorant.
Odors that are mostly perceived as "pleasant":
Odors that are mostly perceived as "unpleasant":
The perception of odors is also very much dependent upon circumstance. The odor of cooking processes may be agreeable while cooking, but not after the meal when one is full. It is also culturally dependent; what smells great to you may be quite unpleasant for your neighbors.
The study of odors is a growing field, but is a complex and difficult one. The human olfactory system can detect many thousands of scents based on only very minute airborne concentrations of a chemical. The sense of smell of many animals is even better. Some fragrant flowers give off odor plumes that move downwind, and are detectable by bees more than a kilometer away.
Pheromones are odors that are deliberately used for communication. A female moth may release a pheromone that can entice a male moth from several kilometers. Honeybee queens constantly are releasing pheromones that regulate the activity of the hive. Workers can release such odors to call other bees into an appropriate cavity when a swarm moves in, or to "sound" an alarm when the hive is threatened.
There are hopes that smelling machines could do everything from test perfumes to help detect cancer, but as yet artificial noses are still quite inadequate.