Saint Ulmo's fire vs arc

Saint Ulmo's fire and arc both are nouns.

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

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

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

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