invariable

1. not comparableNot variable; unalterable; uniform; always having the same value.

   Physical laws which are invariable.

   1860, Isaac Taylor, Ultimate Civilisation

   2 more examples

   “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, →OCLC

   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.

   1957 October 28, Cecil J. Allen, “British Locomotive Practice and Performance”, in Railway Magazine, page 714

2. not comparablemathematicsConstant.
3. broadlynot comparableor a grammatical classgrammarThat cannot undergo inflection, conjugation or declension.

   The French adjective marron ‘brown’ is invariable: it does not take the usual s in the plural.

   Synonyms: uninflectable

plural invariables

1. Something that does not vary; a constant.