will-o'-the-wisp

US /ˌwɪl.oʊ.ðəˈwɪsp/
UK /ˌwɪl.oʊ.ðəˈwɪsp/
"will-o'-the-wisp" picture
1.

a faint, flickering light seen at night over marshy ground, believed to be caused by combustion of natural gases

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The hikers reported seeing a mysterious will-o'-the-wisp in the swamp.
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Folklore often describes the will-o'-the-wisp as leading travelers astray.
:
ignis fatuusspooklightghost light
2.

something that is impossible or difficult to achieve or catch

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Chasing perfection can be a will-o'-the-wisp.
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His promise of a quick solution turned out to be a mere will-o'-the-wisp.
:
illusionchimeradelusionphantommirage
:
wispJell-Otam-o'-shanter