The United States Navy (USN) is the maritime service branch of the United States Armed Forces and is designated as the navy of the United States in the Constitution. With 290 combat vessels, it is the world's second largest navy, behind the People's Liberation Army Navy, and by far the largest by displacement, at 4.5 million tons in 2021. It has the world's largest aircraft carrier fleet, with eleven in service, one undergoing trials, two new carriers under construction, and six other carriers planned as of 2024. The Navy is a part of the Department of Defense and is one of six armed forces and eight uniformed services of the United States.