Essex /ˈɛsɪks/ is a county in the East of England immediately north-east of London and is one of the home counties. It borders the counties of Suffolk and Cambridgeshire to the north, Hertfordshire to the west, Kent across the estuary of the River Thames to the south and London to the south-west. The county town is Chelmsford, which is the only city in the county.

Essex occupies the eastern part of the former Kingdom of Essex, which existed before it and the other Anglian and Saxon kingdoms united to make England a single nation state. As well as rural areas, the county also includes London Stansted Airport, the new towns of Basildon and Harlow, Lakeside Shopping Centre, the port of Tilbury and the borough of Southend-on-Sea.

The name Essex originates in the Anglo-Saxon period of the Early Middle Ages and has its root in the Anglo-Saxon (Old English) name Ēastseaxe ("East Saxons"), the eastern kingdom of the Saxons who had come from the continent and settled in Britain (cf. Middlesex, Sussex and Wessex) during the Heptarchy. Originally recorded in AD 527, Essex occupied territory to the north of the River Thames, incorporating all of what later became Middlesex (which probably included Surrey) and most of what later became Hertfordshire. Its territory was later restricted to lands east of the River Lea.

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