Invariable

In Toki Pona, all words are invariable.

A conjunction is an invariable word that links two ideas.

In Latin, the numbers "unus" and "duus" are inflected, but every other number word is invariable.

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.

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