In economics, unit elasticity occurs when quantity changes by the same percentage as price.
For price elasticity of demand, an elasticity magnitude of one means a 10% price change produces a 10% change in quantity demanded in the opposite direction. Economists call this unit elastic demand. For price elasticity of supply, a 10% price change produces a 10% change in quantity supplied in the same direction.
The percentage comparison matters because it avoids relying on measurement units. A one-dollar change has different significance for a cheap product and an expensive one, while percentage changes make comparisons more meaningful.
For a straight-line demand curve, elasticity is not the same at every point: demand is more elastic near the upper, high-price portion and less elastic near the lower portion. At a unit-elastic point on a demand curve, total revenue is locally maximized in the standard model. Unit elasticity is therefore different from perfectly elastic demand, where the elasticity magnitude is infinite.