Saint Elmo's fire vs arc

Saint Elmo's fire and arc both are nouns.

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

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

Saint 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 Saint Elmo's fire include discharge, electric arc, electric discharge, spark.

Saint 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

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