town and borough (district), administrative and historic county of Essex, England, just outside the northeastern border of Greater London. The borough of Brentwood is to a considerable extent residential, with some light industry, but it extends into the farmlands of the Essex countryside and London's Green Belt. Brentwood became an assize town when judges first went on circuit, and the Elizabethan assize house remains. Brentwood School was founded in 1557. Area 59 square miles (153 square km). Pop. (2001) town, 47,593; borough, 68,426.