demand

In economics, demand is the quantity of a good that consumers are willing and able to purchase at various prices during a given time. 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 particular price and a particular time period since demand is a flow concept. Flow is any variable which is expressed per unit of time. Demand thus does not refer to a single isolated purchase, but a continuous flow of purchases.

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