Plainfield, city (1990 pop. 46,567), Union co., NE N.J.; settled 1684 by Friends, inc. as a city 1869. Formerly a residential city in the New York metropolitan area, it has become the urban center of 10 closely allied municipalities, with diversified industries, including printing and the manufacture of chemicals, machinery, and electronic and transportation equipment. Among the several 18th-century buildings remaining are a Friends' meetinghouse (1788), the Martine house (1717), and the Nathaniel Drake House (1746), known as Washington's Headquarters. Nearby Washington Rock, overlooking the Hudson River, is reputed to be the vantage point from which George Washington watched British troop movements.