'Tax Buoyancy' measures the responsiveness of tax revenue growth to changes in GDP, whereas 'Tax Elasticity' measures the responsiveness of tax revenue to discretionary changes in the tax rate or structure.

Tax Buoyancy is a broader macroeconomic metric; if buoyancy is > 1, tax revenues are growing faster than the economy without any explicit policy changes (due to better compliance or progressive brackets). Tax Elasticity isolates the pure impact of a specific policy shift (like cutting the corporate tax rate) on revenue collection, holding GDP constant.