The meaning of Will-o-the-wisp

idiom

A pale light sometimes seen at night over marshy ground

noun

A phosphorescent light seen hovering or floating at night on marshy ground, thought to result from the combustion of natural gases.

Usage examples:

Pale blue light, the colour of egewe's hair or a will-o'-the-wisp, filled the room.

noun

Something that is impossible to get or achieve

Usage examples:

Full employment is the will-o'-the-wisp that politicians have been chasing for decades.