WordNet sense 1 (persistent in occurrence and unvarying in nature):

constant, unvarying, invariant

WordNet sense 2 (not liable to or capable of change):

invariable

WordNet sense 3 (not subject or susceptible to change or variation in form or quality or nature):

immutable

WordNet sense 4 (remaining the same for indefinitely long times):

unalterable

WordNet sense 5 (not changeable or subject to change):

unchangeable