invariable

Etymology

in- + variable

adj

1. Not variable; unalterable; uniform; always having the same value.
   Physical laws which are invariable. 1860, Isaac Taylor, Ultimate Civilisation
2. (mathematics) Constant.
3. (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.

noun

1. Something that does not vary; a constant.