Found:

Theme vowel

MORPHOLOGY: morphological marker which indicates which is the declensional or conjugational class that a word belongs to. EXAMPLE: in Ancient Greek, nouns are grouped into three declensional classes. If a noun belongs to the first declension, the root is followed by the theme vowel -a-, and if it belongs to the second declension the root is followed by -o-: môus+a+n (acc.sg.), môus+a+y (nom.pl.) 'Muse', log+o+n (acc.sg.), log+o+y (nom.pl.) 'word'. Another term is 'extension vowel'.

LIT.	Goodwin, W. (1894) Spencer, A. (1991)