Trinovantum, in medieval British legend, is the name mythically given to London in earliest times. According to Geoffrey of Monmouth's Historia Regum Britanniae (1136) it was founded by the exiled Trojan Brutus, who called it Troia Nova ("New Troy"), which was gradually corrupted to Trinovantum. It was later rebuilt by King Lud, who named it Caer Lud ("Lud's Fort") after himself, and this name became corrupted to Kaer Llundain, and finally London. This legend is part of the Matter of Britain.
The name Trinovantum derives from the Iron age tribe of the Trinovantes, who lived in Essex, Suffolk and part of Greater London, and who are mentioned by Julius Caesar in his account of his expeditions to Britain in 55 and 54 BC. In a later account of these expeditions by Orosius, they are referred to as civitas Trinovantum, "the nation of the Trinovantes", with Trinovantum in this case being in the genitive plural. However, as civitas can also mean "city" and Latin neuter nouns often end in -um in the nominative singular, this phrase was misinterpreted by Geoffrey or his sources as "the city Trinovantum".
In Roman times the city was known by the name Londinium, which appears to be cognate with Llundain and London. The modern view, backed by the archaeological evidence, is that there was no city on the site of the modern City of London prior to the Roman foundation of Londinium soon after the Claudian invasion of 43 AD. The area was almost certainly open countryside, though people had been passing through for thousands of years.