ignis fatuus
Concise Oxford English Dictionary © 2008 Oxford University Press:
ignis fatuus /ˌɪgnɪs ˈfatjʊəs/
▶noun (pl. ignes fatui /ˌɪgniːz ˈfatjʊʌɪ, ˌɪgneɪz, ˈfatjʊiː/) a will-o'-the-wisp.
– origin C16: mod. L., lit. ‘foolish fire’ (because of its erratic movement).
'ignis fatuus' also found in these Oxford entries:

