Not capable of being changed or altered:
— Unalterable resolve.
— An unalterable ground rule.

synonym: inalterable.

Roget 150: unchangeable, immutable; unaltered, unalterable; not to be changed, constant; permanent etc. 141; invariable, undeviating; stable, durable; perennial etc. (diuturnal) 110^†.    fixed, steadfast, firm, fast, steady, balanced; confirmed, valid; fiducial^†; immovable, irremovable, riveted, rooted; settled, established etc. v.; vested; incontrovertible, stereotyped, indeclinable.    tethered, anchored, moored, at anchor, on a rock, rock solid, firm as a rock; firmly seated, firmly established etc. v.; deep-rooted, ineradicable; inveterate; obstinate etc. 606.    transfixed, stuck fast, aground, high and dry, stranded.    [movable object rendered unmovable] stuck, jammed; unremovable; quiescent etc. 265; deterioration etc. 659.    indefeasible, irretrievable, intransmutable^†, incommutable^†, irresoluble^†, irrevocable, irreversible, reverseless^†, inextinguishable, irreducible; indissoluble, indissolvable^†; indestructible, undying, imperishable, incorruptible, indelible, indeciduous^†; insusceptible, insusceptible of change.    ... show more

Of a sentence; that cannot be changed.

Remaining the same for indefinitely long times.

synonym: changeless.