Immutable, a.
Etymology: [L. immutabilis; pref. im- not + mutabilis mutable. See Mutable.]
Not mutable; not capable or susceptible of change; unchangeable; unalterable. That by two immutable things, in which it was impossible for God to lie, we might have a strong consolation. Heb. vi. 18. Immutable, immortal, infinite, Eternal King. Milton. -- Im*mu"ta*ble*ness, n. -- Im*mu"ta*bly, adv.
Shades of meaning
Fernald has no entry of its own for immutable; he lists it under permanent.
He lists it without comment; the entries above explain its neighbors.