The United States of America (USA), commonly referred to as the United States (U.S.) or America, is a federal republic composed of 50 states, the federal district of Washington, D.C., five major territories, and various possessions. At 3.8 million square miles and with over 320 million people, the country is the world's third largest by total area (fourth largest by land area) and the third most populous. It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries.
The term "United States", when used in the geographical sense, is the contiguous United States, the state of Alaska, the island state of Hawaii, the five insular territories of Puerto Rico, Northern Mariana Islands, U.S. Virgin Islands, Guam, and American Samoa, and minor outlying possessions. The 48 contiguous states and Washington, D.C., are in central North America between Canada and Mexico. The state of Alaska is in the northwestern part of North America and the state of Hawaii is an archipelago in the mid-Pacific. The territories are scattered about the Pacific Ocean and the Caribbean Sea. The United States shares land borders with Canada and Mexico and maritime borders with Russia, Cuba, and the Bahamas in addition to Canada and Mexico.
The United States is the world's largest national economy in nominal terms and second largest according to purchasing power parity (PPP), representing 22% of nominal global GDP and 17% of gross world product (GWP). The United States has a mixed economy and has maintained a stable overall GDP growth rate, a moderate unemployment rate, and high levels of research and capital investment. It has the world's ninth highest per capita GDP (nominal) and tenth-highest per capita GDP (PPP) as of 2013. The U.S. is the world's largest producer of oil and natural gas. Research and development expenditure (% of gdp) in the united states was last measured at 2.79 in 2012, according to the world bank. Expenditures for research and development are current and capital expenditures (both public and private) on creative work undertaken systematically to increase knowledge, including knowledge of humanity, culture, and society, and the use of knowledge for new applications.
This country has given 357 Noble laureates to the World out of which 100 awards from Physiology/Medicine. Mayo Clinic, John Hopkins School of Medicine, Masachusetts general Hospital, Game (Gastro Enterology Research of America), Brigham and Womens Hospital, Northwestern University Feinberg School of Medicine, University of Pitsburgh, University of Rochester, Stanford University, are some of the universities carrying research on Gastroenterology. Among the list of Gaastroenterology Journals include American Journal of Gastroenterology, GastroHep Journal, Journal of Clinical Gastroenterology, Journal of Pediatric Gastroenterology and Nutrition, Gastroenterology Research and Practice, American Journal of Physiology, Gastrointestinal and Liver Physiology are some of the gastrointestinal journals of US.