The United States of America (USA), commonly known as the United States (U.S.) or America, is a federal republic composed of 50 states, a federal district, five major self-governing territories, and various possessions. Forty-eight of the fifty states and the federal district are contiguous and located in North America between Canada and Mexico. The state of Alaska is in the northwest corner of North America, bordered by Canada to the east and across the Bering Strait from Russia to the west. The state of Hawaii is an archipelago in the mid-Pacific Ocean. The U.S. territories are scattered about the Pacific Ocean and the Caribbean Sea and encompass nine time zones. The extremely diverse geography, climate and wildlife of the United States make it one of the world's 17 megadiverse countries.
At 3.8 million square miles (9.8 million km2) and with over 324 million people, the United States is the world's third- or fourth-largest country by total area, third-largest by land area, and the third-most populous. It is one of the world's most ethnically diverse and multicultural nations, and is home to the world's largest immigrant population. The capital is Washington, D.C., and the largest city is New York City; nine other major metropolitan areas—each with at least 4.5 million inhabitants—are Los Angeles, Chicago, Dallas, Houston, Philadelphia, Miami, Atlanta, Boston, and San Francisco.
The United States is a highly developed country, with the world's largest economy by nominal GDP and second-largest economy by PPP. Though its population is only 4.3% of the world total, Americans hold nearly 40% of the total wealth in the world. The United States ranks among the highest in several measures of socioeconomic performance, including average wage, human development, per capita GDP, and productivity per person. While the U.S. economy is considered post-industrial, characterized by the dominance of services and knowledge economy, the manufacturing sector remains the second-largest in the world. Accounting for approximately a quarter of global GDP and a third of global military spending, the United States is the world's foremost economic and military power. The United States is a prominent political and cultural force internationally, and a leader in scientific research and technological innovations.
