Demand

In economics, demand is the quantity of a good consumers are willing and able to purchase at various prices over a given period. In economics, "demand" for a commodity is not the same thing as "desire" for it. It refers to both the desire to purchase and the ability to pay for a commodity.

Demand is always expressed in relation to a range of prices and a particular time period since demand is a flow concept. A flow is any variable expressed per unit of time. Demand refers not to a single isolated purchase, but to a continuous flow of purchases.