Definition of alomorph
A variant phonological representation of a morpheme; "the
final sounds of `bets' and `beds' and `horses' and `oxen'
are allomorphs of the English plural morpheme"
For those without ancient languages... greek and latin roots. Basically morph is shape, allo is "different" or "another".
Another definition for allomorph seems a bit closer to Larry's reply: "An allomorph is one of two or more complementary morphs which manifest a morpheme in its different phonological or morphological environments."
So, I guess in the computer context, an allomorph is an object that manifests differently depending on the context.