locum vs reliever

locum and reliever both are nouns.

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

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

locum: 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 include backup, backup man, fill-in, relief, stand-in, substitute.

locum (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