The United States of America (USA), commonly referred to as the United States (U.S.) or America, is a federal republiccomposed of 50 states, the federal district of Washington DC, five major territories, and various possessions. The 48 contiguous states and Washington DC, 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. 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, andminor outlying possessions. 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' GDP was estimated to be $17.914 trillion as of Q2 2015. 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. Its seven largest trading partners are Canada, China, Mexico, Japan, Germany, South Korea, and the United Kingdom. The US has abundant natural resources, a well-developed infrastructure, and high productivity. It has the world's ninth-highest per capita GDP (nominal) and tenth-highest per capita GDP (PPP) as of 2013. It has been the world's largest national economy (not including colonial empires) since at least the 1890s. The U.S. is the world's largest producer of oil and natural gas.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, American Society of Hematology, Thomas Jefferson University, Yale school of Medicine,Dana Farber Cancer Institute,Boston Childrens Hospital are some of the research institutes carryingout research in US. Among the list of Hematology Journals in US American Journal of Hematology, Blood Cancer Journal, Internet Journal of Hematology, Immunohematology, Bllod cells, Molecules and Diseaeses are some of the Hematology Journals of US.