Ossining (ŏsˈənĭng) [key], village (1990 pop. 22,582), Westchester co., SE N.Y., on the Hudson River; settled c.1750, inc. 1813 as Sing Sing, renamed 1901. Mainly residential, Ossining produces medical instruments and pharmaceuticals. It is the site of Sing Sing state prison (built 1825–28), once known for its extreme discipline, later a leader in prison reform. Maryknoll, headquarters of the Catholic Foreign Mission Society, is nearby. Part of the Old Croton Aqueduct is open to visitors.