In Breton, which belongs to the Brythonic branch of the Insular Celtic languages, along with Welsh and Cornish, "on sea" is war vor (Welsh ar fôr, "f" being voiced and pronounced like English "v"), but the older form arvor is used to refer to the coastal regions of Brittany, in contrast to argoad (ar "on/at", coad "forest" ) for the inland regions. The cognate modern usages suggest that the Romans first contacted coastal people in the inland region and assumed that the regional name Aremorica referred to the whole area, both coastal and inland.
Pliny the Elder, in his Natural History (4.17.105), claims that Armorica was the older name for Aquitania and states Armorica's southern boundary extended to the Pyrenees. Taking into account the Gaulish origin of the name, that is perfectly correct and logical, as Aremorica is not a country name but a word that describes a type of geographical region, one that is by the sea. Pliny lists the following Celtic tribes as living in the area: the Aedui and Carnuteni as having treaties with Rome; the Meldi and Secusiani as having some measure of independence; and the Boii, Senones, Aulerci (both the Eburovices and Cenomani), the Parisii, Tricasses, Andicavi, Viducasses, Bodiocasses, Veneti, Coriosvelites, Diablinti, Rhedones, Turones, and the Atseui.
Trade between Armorica and Britain, described by Diodorus Siculus and implied by Pliny was long-established. Because, even after the campaign of Publius Crassus in 57 BC, continued resistance to Roman rule in Armorica was still being supported by Celtic aristocrats in Britain, Julius Caesar led two invasions of Britain in 55 and 54 in response. Some hint of the complicated cultural web that bound Armorica and the Britanniae (the "Britains" of Pliny) is given by Caesar when he describes Diviciacus of the Suessiones, as "the most powerful ruler in the whole of Gaul, who had control not only over a large area of this region but also of Britain" Archaeological sites along the south coast of England, notably at Hengistbury Head, show connections with Armorica as far east as the Solent. This 'prehistoric' connection of Cornwall and Brittany set the stage for the link that continued into the medieval era. Still farther East, however, the typical Continental connections of the Britannic coast were with the lower Seine valley instead.
Archaeology has not yet been as enlightening in Iron-Age Armorica as the coinage, which has been surveyed by Philip de Jersey.
Under the Roman Empire, Armorica was administered as part of the province of Gallia Lugdunensis, which had its capital in Lugdunum, (modern day Lyon). When the Roman provinces were reorganized in the 4th century, Armorica (Tractus Armoricanus et Nervicanus) was placed under the second and third divisions of Lugdunensis. After the legions retreated from Britannia (407) the local elite there expelled the civilian magistrates in the following year; Armorica too rebelled in the 430s and again in the 440s, throwing out the ruling officials, as the Romano-Britons had done. At the Battle of the Catalaunian Plains in 451 a Roman coalition led by General Flavius Aetius and the Visigothic King Theodoric I clashed violently with the Hunnic alliance commanded by King Attila the Hun. Jordanes lists Aëtius' allies as including Armoricans and other Celtic or German tribes (Getica 36.191).