This document defines elasticity as the percentage change in one variable due to a 1% change in another variable. It discusses price elasticity of demand, which is the responsiveness of quantity demanded to a change in price. Perfectly elastic, relatively elastic, unitary, relatively inelastic, and perfectly inelastic demands are defined based on the elasticity value. Point and arc elasticities are introduced as ways to measure elasticity. Determinants of price elasticity like availability of substitutes and necessities vs luxuries are outlined. Income elasticity and cross elasticity are also defined. An example of calculating price elasticity using a demand equation is provided.