What people or businesses anticipate will happen, especially in terms of markets and prices. Expectations are a key determinant of demand and of supply and are assumed to be constant when the demand and supply curves are constructed. As a determinant of demand and supply, changes in expectations cause shifts of the demand and supply curves -- a " change in demand" or a " change in supply." Expectations are also important to the study of inflation and the aggregate market as people and firms make decisions and behave according to what they anticipate will happen in the future.