eidolon [ahy-doh-luhn]

noun:
1 a phantom; apparition
2 an ideal

Examples:

The gods send an eidolon (an image of Helen, made of air) to Troy instead. The war is fought over the eidolon and the city is destroyed. The Greeks finally reclaim eidolon-Helen, whereupon she disappears into the air from which she was made. (Natalie Haynes, Helen of Troy: the Greek epics are not just about war - they're about women, The Guardian, November 2019)

There the eidolon sits, flickering like a neon light deep in the library stacks, swinging her legs atop the sliding shelves where the crumbly books by dead men wait in dusty darkness for the touch of human hands. (Lauren Groff, Judith Shakespeare, Grinning Literary Ghost: Lauren Groff on the Nuances of A Room of One’s Own, Literary Hub, January 2025)

A dark wisp of smoke - Percy guessed it must be an eidolon - seeped into a Cyclops, made the monster hit himself in the face, then drifted off to possess another victim. (Rick Riordan, The House of Hades)

You have never seen Mr Wakem before, and are possibly wondering whether he was really as eminent a rascal, and as crafty, bitter an enemy of honest humanity in general, and of Mr Tulliver in particular, as he is represented to be in that eidolon or portrait of him which we have seen to exist in the miller's mind. (George Eliot, The Mill on the Floss)

Origin:
1801, 'a shade, a specter,' from Greek eidolon 'appearance, reflection in water or a mirror,' later 'mental image, apparition, phantom,' also 'material image, statue, image of a god, idol,' from eidos 'form, shape'. By 1881 in English as 'a likeness, an image.' (Online Etymology Dictionary)