indeclinable

/IH0 N D IH0 K L AY1 N AH0 B AH0 L/
noun
  1. 1

    (grammar) A word that is not grammatically inflected.

adjective
  1. 1

    That one cannot decline; unavoidable.

  2. 2

    (grammar, of a word) Not grammatically declinable.

Synonyms

Source: https://en.wiktionary.org/wiki/indeclinable