The Collaborative International Dictionary
Imparisyllabic
Imparisyllabic \Im*par"i*syl*lab"ic\, a. [L. impar unequal + E. syllabic: cf. F. imparisyllabique.] (Gram.) Not consisting of an equal number of syllables; as, an imparisyllabic noun, one which has not the same number of syllables in all the cases; as, lapis, lapidis; mens, mentis.
Wiktionary
imparisyllabic
a. Not having the same number of syllables in all its inflections (especially of Greek or Latin nouns); for example, the Latin word pars, which has the genitive form partis. n. An imparisyllabic noun.