Lindsey was an ancient Anglo-Saxon territory documented from the seventh century. It may once have been an independent kingdom, though no royal line can identified with confidence. Lindsey was the territory of the Danish borough of Lincoln and from the eleventh century onwards formed the northern part of the county of Lincoln, with Hollandand Kesteven as its southern appendages. The greater importance of Lindsey was signified by the fact that the name was frequently used as a synonym for the county of Lincolnshire in Domesday Book.
For more detail, see Cyril R. Hart, The Danelaw (1992); and Peter H. Sawyer, Anglo-Saxon Lincolnshire (1998).