Brittany is the top-left, tongue-shaped, bit of France.
When the Romans were there, they called it Armorica and it was inhabited by Gauls. By the 4th century tribesmen from Britain had begun to cross the Channel to settle there, a flow which accelerated during the 5th century as Roman troops and authority were withdrawn from Britain, and Anglo-Saxons entered the country from the east.
To distinguish it from Britain proper, the region acquired the name of Brittany or Lesser Britain.
As time passed the region divided into three kingdoms -- Domnonia, Cornouaille and Bro Waroch (the first two taking their names from the Roman for "Devon" and the French for "Cornwall") -- which were later amalgamated once more, to form the Duchy of Brittany.