fill-in vs locum tenens

fill-in and locum tenens both are nouns.

Word	Noun	Adjective	Verb	Adverb
fill-in	Yes	No	No	No
locum tenens	Yes	No	No	No

As nouns, locum tenens is a hyponym of fill-in; that is, locum tenens is a word with a more specific, narrower meaning than fill-in:

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

Other hyponyms of fill-in include locum, double, alternate, replacement, surrogate.

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