morphophonemics
noun
mor·pho·pho·ne·mics ˌmȯr-fō-fə-ˈnē-miks 
plural in form but singular in construction
1
: a study of the phonemic differences between allomorphs of the same morpheme
2
: the distribution of allomorphs in one morpheme
3
: the structure of a language in terms of morphophonemics
Word History
First Known Use
1938, in the meaning defined at sense 1