euhemerism (n.)

1846, "the method of regarding myths as glorified accounts of actual events or persons," with -ism + name of Euhemerus, Greek philosopher of Sicily (4c. B.C.E.), who wrote "Iera Anagraphe," in which he maintained the Greek deities actually were historical mortals. His name is literally "good day," from eu "well, good" (see eu-) + hēmera "day" (from PIE *Hehmer "day"). Related: Euhemerist; euhemeristic.