stand-in vs locum

stand-in and locum both are nouns.

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

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

stand-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 stand-in include locum tenens, double, alternate, replacement, surrogate.

stand-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