invariable
/ɪnˈvɛə.ɹi.ə.bl̩/
noun
Meaning
Something that does not vary; a constant.
adjective
Meaning
Not variable; unalterable; uniform; always having the same value.
Constant.
(by extension, grammar, of a word, or a grammatical class) That cannot undergo inflection, conjugation or declension.
"The French adjective marron (brown) is invariable: it does not take the usual s in the plural."