Economists use the term demand to refer to the amount of some good or service consumers are willing and able to purchase at each price. Demand is based on needs and wants – a consumer may be able to differentiate between a need and a want, but from an economist’s perspective, they are the same thing. Demand is also based on ability to pay. If you can’t pay for it, you have no effective demand.
What a buyer pays for a unit of the specific good or service is called the price (P). The total number of units purchased at that price is called the quantity demanded (Q). A rise in the price of a good or service almost always decreases the quantity of that good or service demanded. Conversely, a fall in price will increase the quantity demanded. When the price of a gallon of gasoline goes up, for example, people look for ways to reduce their consumption by combining several errands, commuting by carpool or mass transit, or taking weekend or vacation trips closer to home. Economists call this inverse relationship between price and quantity demanded the law of demand. The law of demand assumes that all other variables that affect demand are held constant (“ceteris paribus”).