If you have a sound change or allophony rule such as:
X -> Y / _Z
Is there a common term for
Y is often called a reflex, though that term is used not just for sounds but also for morphemes or words: e.g. you can say that the French reflex of Latin /k/ before /a/ is /ʃ/, or that the French reflex of Latin caper is chèvre. Proto-phoneme is sometimes used for X, but this is less common (probably because there's less often a practical need for such a term).