Teesside, former county borough, NE England, at the mouth of the Tees River. Teesside was created in 1968 by the merger of the county borough of Middlesbrough; the municipal boroughs of Stockton-on-Tees, Thornaby-on-Tees, and Redcar (except for a very small part); most of the urban districts of Billingham and Eston; smaller portions of the urban districts of Guisborough, Saltburn, and Marske-by-the-Sea; and parts of the rural districts of Stockton and Stokesley. Boulby Cliff, the highest cliff on the English coastline, was within its borders. In 1974, however, Teesside split into the boroughs of Stockton-on-Tees, Middlesbrough and Langbaurgh (now Redcar and Cleveland). Before iron was discovered there in 1831, Middlesbrough was a coal port. Stockton-on-Tees developed industrially after the opening of the Stockton and Darlington Railway (the first important railroad in Great Britain) in 1825. Thomas Sheraton, the furniture designer, was born there. The name is also spelled Teeside.