Westchester County is a county in the U.S. state of New York. It is the second-most populous county on the mainland of New York, after the Bronx, and the most populous county in the state north of New York City. Situated in the Hudson Valley, Westchester covers an area of 450 square miles, consisting of six cities, 19 towns, and 23 villages. Established in 1683, Westchester was named after the city of Chester, England. The county seat is the city of White Plains, while the most populous municipality in the county is the city of Yonkers, with an estimated 200,807 residents in 2016.
Westchester County is one of the centrally located counties within the New York metropolitan area. The county is positioned with New York City, plus Nassau and Suffolk counties (on Long Island, across Long Island Sound), to its south; Putnam County to its north; Fairfield County, Connecticut to its east; and Rockland County and Bergen County, New Jersey across the Hudson River to the west. Westchester was the first suburban area of its scale in the world to develop, due mostly to the upper-middle-class development of entire communities in the late 19th century and the subsequent rapid population growth.