The relationship between the prices that consumers are willing and able to pay for various quantities of a good or service for a given time period, all other things constant.

For example to see how much Coke consumers want to drink depends on the price of Coke. When the price of Coke rises some people drink less of it, perhaps switching completely to other caffeinated beverages such as tea or coffee. In general, the quantity of Coke, or of any good or service that people want to buy, depends on the price. The higher the price, the less of the good or service people want to purchase; alternatively, the lower the price, the more they want to purchase.