locum tenens vs reliever

locum tenens and reliever both are nouns.

Word	Noun	Adjective	Verb	Adverb
locum tenens	Yes	No	No	No
reliever	Yes	No	No	No

As nouns, reliever is a hypernym of locum tenens; that is, reliever is a word with a broader meaning than locum tenens:

locum tenens: someone (physician or clergyman) who substitutes temporarily for another member of the same profession
reliever: someone who takes the place of another (as when things get dangerous or difficult)

Other hypernyms of locum tenens include backup, backup man, fill-in, relief, stand-in, substitute.

locum tenens (noun)	reliever (noun)
someone (physician or clergyman) who substitutes temporarily for another member of the same profession	a pitcher who does not start the game
	someone who takes the place of another (as when things get dangerous or difficult)
	a person who reduces the intensity (e.g., of fears) and calms and pacifies