Baldwin. 1 Uninc. city (1990 pop. 22,719), Nassau co., SE N.Y., on the south shore of Long Island, on Baldwin Bay; settled 1640s. A fishing center and summer resort, it has varied manufactures. 2 Borough (1990 pop. 21,923), Allegheny co., SW Pa., a suburb just S of Pittsburgh, on the Monongahela River, in a bituminous coal region; inc. 1952.