Hatfield, town (1991 pop. 33,174), Hertfordshire, SE England. Hatfield was designated one of the new towns in 1948 to alleviate overpopulation in London. The plans for this new town were coordinated with those of nearby Welwyn Garden City. Aircraft, engineering, and electronics manufacture are the town's most notable industries. Hatfield House, the Jacobean mansion built by the 1st earl of Salisbury, and Hatfield Polytechnic College are there.