The percentage change in quantity divided by a percentage change in the price. Answers the question: How much will the demand for my product decrease if I raise prices by 10%?
Elasticity of Demand
The relative stability of a security's or product's price in the face of increased or decreased demand. Elastic securities or products have prices that move as independently as possible from changes in demand. In securities, elasticity is strongly influenced by the number of shares outstanding; if a company has many shares outstanding, a large order to buy or sell them is less likely to affect the price as strongly as a similar order for a company with comparatively few shares outstanding. In other products, elasticity largely comes from whether a given product is considered a necessity or a luxury. A "necessary" product is likely to be more elastic. See also: Income Elasticity of Demand.