Synonyms for Eonian (same or very similar meaning)
WordNet sense 1 (of or relating to a geological eon (longer than an era)):
aeonianWordNet sense 2 (continuing forever or indefinitely):
ageless, eternal, everlasting, perpetual, unceasing, unendingWordNet sense 3 (continuing or enduring without marked change in status or condition or place):
lasting, permanentFrom the ODE community, based on WordNetadd/edit