An eponym.So I couldn't figure out the roots!
Euhemerism
(n.) The theory, held by Euhemerus, that the gods of mythology were but deified mortals, and their deeds only the amplification in imagination of human acts.

Sounds like baloney to me. Some mortals were indeed deified,
but not Zeus and colleagues.

Some that we know to have been mortals were run through a process that is said to have made them gods, e.g., many of the Roman Emperors. What I think Euhemerus was saying is that those we think of as gods, e.g., Zeus, were mortals from long ago, of whom we have no other knowledge than the tales told about them and whom we have raised to the level of godhood by tradition.

Yup, that's how I've seen the word used.