1. immutable (vs. mutable), changeless

usage: not subject or susceptible to change or variation in form or quality or nature; "the view of that time was that all species were immutable, created by God"

2. changeless, constant, invariant, unvarying, invariable (vs. variable)

usage: unvarying in nature; "maintained a constant temperature"; "principles of unvarying validity"

3. changeless, unalterable, unchangeable (vs. changeable)

usage: remaining the same for indefinitely long times

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See also: changeless (Dictionary)