locum tenens vs stand-in

locum tenens and stand-in both are nouns.

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

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

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

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

locum tenens (noun)	stand-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)