Armorica or Aremorica is the name given in ancient times to the part of Gaul that includes the Brittany peninsula and the territory between the Seine and Loire rivers, extending inland to an indeterminate point and down the Atlantic coast. The toponym is based on the Gaulish phrase are-mori "on/at [the] sea", made into the Gaulish place name Aremorica (*are-mor-ika ) "Place by the Sea". The suffix -ika was first used to create adjectival forms and then, names (See regions as Pays d'Ouche < Utica, Perche < Pertica ). The original designation was vague, including a large part of what became Normandy in the 10th century and, in some interpretations, the whole of the coast down to the Pyrenees. Later, the term became restricted to Brittany.

In Breton (which with Welsh and Cornish belongs to the Brythonic branch of Insular Celtic languages), "on [the] sea" is war vor (Welsh ar y môr), though the older form arvor is used to refer to the coastal regions of Brittany, in contrast to argoad (ar "on/at", coad "forest" [Welsh ar goed (coed "trees")] for the inland regions. These 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.