In ancient Greek literature, an eidolon (plural: eidola or eidolons; Greek εἴδωλον 'image, idol, double, apparition, phantom, ghost') is a spirit-image of a living or dead person; a shade or phantom look-alike of the human form. The concept of Helen of Troy's eidolon was explored both by Homer and Euripides. However, where Homer uses the concept as a free-standing idea that gives Helen life after death, Euripides entangles it with the idea of kleos, one being the product of the other. Both Euripides and Stesichorus, in their respective works concerning the Trojan Horse, claim that Helen was never physically present in the city at all.
Walt Whitman's poem by the same name in 1876 used a much broader understanding of the term, expanded and detailed in the poem. In Whitman's use of the term we can see the use broaden to include the concept of an oversoul composed of the individual souls of all life and expanding to include the Earth itself and the hierarchy of the planets, Sun, stars and galaxy.