Historic county, southeastern England, situated along the River Thames. Its earliest settlements date from 500 BC; Belgic tribes arrived in the 1st century BC, followed by the Romans, who established outposts along the river. It was colonized by Saxons in the early 5th century AD. Situated between the East and West Saxons, it obtained its name (meaning middle Saxons) by AD 704. Middlesex was established as an administrative county in 1888. In 1965 Greater London incorporated parts from the former county.