noun which may take the genitive case particle 'no'
indeclinable
Grammar term for words that do not inflect, such as particles, adverbs, and conjunctions in Japanese.
日本語の助詞は不変化の品詞です。
Japanese particles are indeclinable parts of speech.
indeclinable; particle
From 不 (not) + 変化 (change), literally 'no change', referring to the lack of inflection.