Excess demand and supply

Normal Good

are any goods for which demand increases when income increases, and falls when income decreases but price remains constant, i.e. with a positive income elasticity of demand.

Luxury Good

is a good for which demand increases more than proportionally as income rises, and is a contrast to a "necessity good", for which demand increases proportionally less than income.

Inferior Good

is a good whose demand decreases when consumer income rises (or demand rises when consumer income decreases), unlike normal goods, for which the opposite is observed.

Price Elasticity Of Demand

is a measure used in economics to show the responsiveness, or elasticity, of the quantity demanded of a good or service to a change in its price when nothing but the price changes.

Price Elasticity Of Supply

is a measure used in economics to show the responsiveness, or elasticity, of the quantity supplied of a good or service to a change in its price.

Excess Demand

is a situation in which the demand for a product or service exceeds its supply in a market.

Excess Supply

is a situation in which the quantity of a good or service supplied is more than the quantity demanded, and the price is above the equilibrium level determined by supply and demand.

Necessity Good

is a type of normal good, but the increase is less than proportional to the rise of income, so the proportion of expenditure on these goods falls as income rises.

Superior Good

make up a larger proportion of consumption as income rises, and therefore are a type of normal goods in consumer theory.

Cross Elasticity Of Demand

measures the responsiveness of the quantity demanded for a good to a change in the price of another good, ceteris paribus.

Economic Surplus

refers to two related quantities: consumer surplus and producer surplus