From Classic Encyclopedia 1911
NEUSTRIA, the old name given to the western kingdom of the Franks, as opposed to the eastern kingdom, Austrasia. The most ancient form of the word is Niuster, from niust, which would make the word signify the "most recent" conquests of the Franks. The word Neustria does not appear as early as the Historia Francorum of Gregory of Tours, but is found for the first time in Fredegarius. The kingdom of Chilperic was retrospectively given this name, and in contemporary usage it was given to the kingdom of Clovis II., as opposed to that of Sigebert III., the two sons of Dagobert; and after that, the princes reigning in the West were called kings of Neustria, and those reigning in the East, kings of Austrasia. Under the new Carolingian dynasty, Pippin and Charlemagne restored the unity of the Frankish realm, and then the word Neustria was restricted to the district between the Loire and the Seine, together with part of the diocese of Rouen north of the Seine; while Austrasia comprised only the Frankish dominions beyond the Rhine, perhaps with the addition of the three cities of Mainz, Worms and Spires on the left bank. The districts between Neustria and Austrasia were called Media Francia or simply Francia. In 843 Brittany took from Neustria the countships of Rennes and Nantes; and gradually the term Neustria came to be restricted to the district which was later called Normandy, Dudo of Saint Quentin, who flourished about the year 1000, gives the name Neustria to the lands ceded to Rollo and his followers during the loth century. In the year 1663, the Pere de Moustier gave to his work on the churches and abbeys of Normandy the title of Neustria pia. At the time of Charlemagne, Lombardy was divided into five provinces: Neustria, Austrasia, Aemilia, Littoraria marls and Tuscia. Austrasia was the name given to eastern Lombardy, and Neustria that given to western Lombardy, the part last occupied by the Lombards.
See F. Bourquelet, "Sens des mots France et Neustrie sous le regime merovingien," in the Bibliotheque de t'ecole des chartes, xxvi. 566-574; Longnon, Atlas historique de la France, both atlas and text. (C. Pr.)