St Elmo's fire
Concise Oxford English Dictionary © 2008 Oxford University Press:
St Elmo's fire /ˈɛlməʊz/
▶noun a phenomenon in which a luminous electrical discharge appears on a ship or aircraft during a storm.
– origin regarded as a sign of protection given by St Elmo, the patron saint of sailors.
'St Elmo's fire' also found in these Oxford entries:

