Definition of "invariable"

invariable

/ɪnˈvɛɹ.i.ə.bl̩/

adjective
not comparable
1. Not variable; unalterable; uniform; always having the same value.
  Quotations
  Physical laws which are invariable.
  1860, Isaac Taylor, Ultimate Civilisation
  “A tight little craft,” was Austin’s invariable comment on the matron; and she looked it, always trim and trig and smooth of surface like a converted yacht cleared for action.
  1907 August, Robert W[illiam] Chambers, chapter IX, in The Younger Set, New York, N.Y.: D. Appleton & Company
2. (mathematics) Constant.
3. (by extension, grammar, of a word, or a grammatical class) That cannot undergo inflection, conjugation or declension.
noun
plural invariables
1. Something that does not vary; a constant.