Trinovant(r)um, in medieval British legend, is the name 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 gradually corrupted to Trinovantum. It was later rebuilt by King Lud, who named it Caer Lud after himself, and this name became corrupted to Kaer Llundain, and finally London. This legend is part of the Matter of Britain. In Roman times it was known by the name Londinium, from which the name London is more usually derived.
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 tribe 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 has been misinterpreted by Geoffrey or his sources as "the city Trinovantum".