elastic demand  demand for which price elasticity is greater than 1.
income elasticity of demand  the percentage change in quantity demanded of one good divided by the percentage change in income.
inelastic demand  demand for which the price elasticity is less than 1.
perfectly elastic demand  demand for which the price elasticity is infinite, indicating an infinite response to a change in the price and therefore a horizontal demand curve.
perfectly elastic supply  supply for which the price elasticity is infinite, indicating an infinite response of quantity supplied to a change in price and thereby a horizontal supply curve.
perfectly inelastic demand  demand for which the price elasticity is zero, indicating no response to a change in price and therefore a vertical demand curve.
perfectly inelastic supply  supply for which the price elasticity is zero, indicating no response of quantity supplied to a change in price and thereby a vertical supply curve.
unit-free measure  a measure that does not depend on a unit of measurement.