What economic measure shows how strongly quantity demanded responds to a change in price?

The economic measure showing how strongly quantity demanded responds to a price change is price elasticity of demand.

Price elasticity of demand compares the percentage change in quantity demanded with the percentage change in price. Because price and quantity demanded usually move in opposite directions, the unadjusted value is normally negative; economists often discuss its absolute value. Demand is called elastic when quantity responds proportionally more than price, and inelastic when it responds proportionally less.

The measure helps explain why a price increase can raise or reduce total revenue. With elastic demand, a higher price tends to reduce revenue because quantity falls relatively sharply. With inelastic demand, a higher price tends to increase revenue, at least within the relevant range.

Elasticity depends on factors such as substitute availability, whether the product is a necessity, the share of income it uses, and the time consumers have to adjust. It is not the same as slope: elasticity is unit-free and can vary along a straight-line demand curve.