reliever vs locum

reliever and locum both are nouns.

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

As nouns, locum is a hyponym of reliever; that is, locum is a word with a more specific, narrower meaning than reliever:

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

Other hyponyms of reliever include locum tenens, double, alternate, replacement, surrogate.

reliever (noun)	locum (noun)
a pitcher who does not start the game	someone (physician or clergyman) who substitutes temporarily for another member of the same profession
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