St. Elmo's fire vs arc

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

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

As nouns, arc is a hypernym of St. Elmo's fire; that is, 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
arc: electrical conduction through a gas in an applied electric field

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

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

St. Elmo's fire (verb)	arc (verb)
	form an arch or curve