invariable

adj

1. (not-comparable)Not variable; unalterable; uniform; always having the same value.

   “Physical laws which are invariable.”

   ““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.”

   “The rebuilt "Merchant Navy" Pacifics have the monopoly of these workings, and Mr. C. K. Dunkley writes that as a result of their reduced water consumption, as compared with the unrebuilt engines, some engine-crews no longer spend time taking water at Southampton Central, the invariable practice previously.”

2. (not-comparable)Constant.
3. (broadly, not-comparable)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.