fill-in vs locum

fill-in and locum both are nouns.

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

As nouns, locum is a hyponym of fill-in; that is, locum 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: someone (physician or clergyman) who substitutes temporarily for another member of the same profession

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

fill-in (noun)	locum (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