The William M. (Mac) Thornberry National Defense Authorization Act for Fiscal Year 2021 is a United States federal law which specifies the budget, expenditures and policies of the U.S. Department of Defense (DoD) for fiscal year 2021. Analogous NDAAs have been passed annually for 59 years. The act is named in honor of Representative Mac Thornberry, who served as either the chair or the ranking member of the House Armed Services Committee. Thornberry retired from Congress at the end of the congressional session.