indeclinable

Without grammatical inflection.

Of or being a word that lacks grammatical inflection though belonging to a form class whose members are usually inflected.

In grammar, not declinable; not varied by declension; showing no variety of form for case, number, or the like.

In grammar, a word that is not declined.

Not declinable; not varied by inflective terminations.

not grammatically inflected, especially if others of its class are usually inflected

A word that is not grammatically inflected.