ETERNAL definition: 1. lasting forever or for a very long time: 2. lasting forever or for a very long time: 3…. Learn more. Definition of eternity noun in Oxford Advanced Learner's Dictionary. Meaning, pronunciation, picture, example sentences, grammar, usage notes, synonyms and more. ETERNITY meaning: 1. time that never ends or that has no limits: 2. a very long time: 3. time that never ends or…. Learn more. Eternity definition: Time without beginning or end; infinite time.