Eureka can function as an interjection or an adjective. An interjection is an ejaculatory utterance that usually lacks grammatical connection (someone who has just made a discovery may yell eureka), and an adjective modifies a noun (the person might describe this discovery as a eureka moment). While epiphany may cover some similar semantic terrain (particularly the sense meaning "an illuminating discovery, realization, or disclosure"), in terms of its function in a sentence, it is a noun.