What is elasticity in the terms of economics?

In economics it refers to a quantitative measurement of the degree of flexibility of something in response to something else. For example, the “elasticity of demand with respect to income” or the “income elasticity of demand” for a product refers to the percentage change in the quantity of the product demanded in response to a 1% change in consumers’ income, or more generally to the ratio of the percentage change in quantity demanded to the percentage change in income.