monoptote
A noun which only occurs in one case.
obsolete
A noun which is found in one case only, is called a Monoptote; if found in two cases, a Diptote; if in three, a Triptote; if in four, a Tetraptote; and if in five, a Pentaptote.
Where it comes from
From Latin monoptotum, from Ancient Greek μονόπτωτος (monóptōtos).
Adapted from the Wiktionary entry monoptote (CC BY-SA 4.0).