Brentwood. 1 Uninc. town (1990 pop. 45,218), Suffolk co., SE N.Y., on central Long Island, in the town of Islip. It is mainly residential, with various light industries. Josiah Warren led (1851) an experiment in communal living in Brentwood. A campus of the New York Institute of Technology is there. 2 Borough (1990 pop. 10,823), Allegheny co., W Pa., a residential suburb of Pittsburgh; inc. 1915.