St. Elmo's fire vs electric arc

St. Elmo's fire and electric arc both are nouns.

Word	Noun	Adjective	Verb	Adverb
St. Elmo's fire	Yes	No	No	No
electric arc	Yes	No	No	No

As nouns, electric arc is a hypernym of St. Elmo's fire; that is, electric arc is a word with a broader meaning than St. Elmo's fire:

St. Elmo's fire: an electrical discharge accompanied by ionization of surrounding atmosphere
electric arc: electrical conduction through a gas in an applied electric field

Other hypernyms of St. Elmo's fire include arc, discharge, electric discharge, spark.

St. Elmo's fire (noun)	electric arc (noun)
an electrical discharge accompanied by ionization of surrounding atmosphere	electrical conduction through a gas in an applied electric field