"The name is from Celtic *aballa-; there is Gaulish avallo = 'poma' in the Vienna Glossary. The modem languages show Welsh afall 'apple-tree', Welsh afal 'apple' and Breton aval, Irish ubhall, abhall. Cognate Germanic words include English apple and others which go back to a common North European base *abl-. Latin nux avellana 'hazel-nut' may involve a borrowing from Celtic, though there was possibly a native Italie représentative of the group (see below).
Romano-British Aballava can hardly refer to a fruit or to a single distinguished tree, although it might (as with other tree-references in place-names) allude to a sacred tree. A more mundane explanation is that the name is a collective 'orchard ' ; Irish abhall is used in this sense in some parts. The name is paralleled in Gaul : Aballo TP = Aballone AI  l604 > Avallon (Yonne, France), and there are other places called Avalon in France. In Italic lands there was Abella  (Campania), described as malifera 'rich in apples' in the Aeneid VII.740.

The name has the derivational suffix -ava (British *-auà), as in Galava, Manavia, and abroad Genava, etc. ; see Holder I.305. It is now represented by Welsh -au, Breton -aou -ou.

IDENTIFICATION. 

The Roman fort at Burgh-by-Sands, Cumberland (NY 3259).

Note. Presumably the Avalon of Arthurian legend has a like origin. The name appears first in Geoffrey of Monmouth, and the equation with Glastonbury was made with the discovery of the pretended tomb of Arthur at the abbey in 1191 : 

Hic iacet sepultus inditus rex Arturius in insula Avalonia. 

This was adopted in Welsh as Ynys Afallon 'Isle of Apples', perhaps a Celtic version of the Greek Hesperides".