locum vs fill-in

locum and fill-in both are nouns.

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

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

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

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

locum (noun)	fill-in (noun)
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)