administrative, geographic, and historic county in eastern England, extending along the North Sea coast from the Humber estuary to the Wash. The administrative, geographic, and historic counties cover slightly different areas. The administrative county comprises seven districts: East Lindsey, West Lindsey, North Kesteven, South Kesteven, South Holland, the borough of Boston, and the city of Lincoln. The geographic county encompasses the entire administrative county, as well as the unitary authorities of North East Lincolnshire and North Lincolnshire. The historic county is nearly coterminous with the geographic county, except for a few small areas on the northwestern fringes of the North Lincolnshire unitary authority that belong to the historic county of Yorkshire.

Lincolnshire contains two prominent upland areas, which cross it from north to south. The more westerly, and narrower, is Lincolnshire Edge, a limestone escarpment rising abruptly on its western side and upon which the city of Lincoln stands. Separated from the Edge by a clay lowland are the Lincolnshire Wolds, an area of rolling chalk hills. Between the Wolds and the coast lies an area known as the Lincoln Marsh; at its southern end it merges into the low-lying area of the Fens, which surrounds the Wash and has been the scene of drainage and reclamation efforts at least since Roman times.

The upland areas of Lincolnshire provided dry defensive sites for prehistoric settlement, which increased in density as trade developed across the North Sea with Europe. A prehistoric salt industry existed on the coast around Ingoldmells. There also was extensive Roman settlement in the county. The main artery of the Roman Ermine Street followed the line of the Lincoln Edge, intersecting with the Fosse Way at Lincoln (then called Lindum Colonia), an important town in Roman times. The Anglo-Saxons, who occupied the area when the Romans departed, penetrated from the sea along the River Trent and established the kingdom of Lindsey (Lindsey, Parts of). Danish influence was also widespread; the county contains two Danish boroughs—Lincoln and Stamford—and many villages established by the Danes.

Large churches, abbeys, and monasteries attest to the prosperity medieval Lincolnshire derived from its agriculture. Its towns, particularly Lincoln and Stamford, thrived on the medieval woolen trade, and wool was a major export of the port of Boston. The draining of the Fens in the 17th century greatly increased the county's agricultural production. As a result of the Industrial Revolution and the shift of population to cities and coalfields, however, the county began to find itself outside the main lines of circulation across England in the 19th century. Partly isolated by the deep indentations of the Humber and the Wash at its northern and southern boundaries, Lincolnshire relied primarily on its agriculture and the processing industries that it brought into being. With its deeply rural countryside and small market towns, the centre of the county presents something of a reminder of rural England in a past era. An exception to this trend is the northern part of the county along the Humber, where railways spurred the growth of ports such as Immingham and Grimsby, as well as the steel-making town of Scunthorpe. The growth of tourism also brought new vitality to the coast. Campgrounds and seaside resorts such as Skegness, Mablethorpe, and Cleethorpes form an almost unbroken line along the North Sea coast except at a few places, most notably Gibraltar Point, where nature reserves have been established.

Grains, sugar beets, and vegetable crops are the main farm products of the county. The main industries are food processing, especially in Lincoln and other central and southern towns, and oil refining, chemicals, and steel production in northern Lincolnshire. The northern ports of Grimsby and Immingham remain among Britain's busiest. Area administrative county, 2,286 square miles (5,921 square km); geographic county, 2,673 square miles (6,922 square km). Pop. (2005 est.) administrative county, 678,700; geographic county, 993,300.