late 14c., "man of superhuman strength or physical courage," from Old French heroe (14c., Modern French héros), from Latin heros (plural heroes) "hero, demi-god, illustrious man," from Greek hērōs (plural hērōes) "demi-god," a variant singular of which was hērōe. This is of uncertain origin; perhaps originally "defender, protector" and from PIE root *ser- (1) "to protect," but Beekes writes that it is "Probably a Pre-Greek word."
Meaning "man who exhibits great bravery" in any course of action is from 1660s in English. Sense of "chief male character in a play, story, etc." first recorded 1690s. Hero-worship is from 1713 in reference to ancient cults and mysteries; of living men by 1830s. In Homer, of the Greeks before Troy, then a comprehensive term used of warriors generally, also of all free men in the Heroic Age. In classical mythology from at least the time of Hesiod (8c. B.C.E.) "man born from a god and a mortal," especially one who had done service to mankind; with the exception of Heracles limited to local deities and patrons of cities.
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Definitions of heroism from WordNet
the qualities of a hero or heroine; exceptional or heroic courage when facing danger (especially in battle);