in-law vs father-in-law

in-law and father-in-law both are nouns.

Word	Noun	Adjective	Verb	Adverb
in-law	Yes	No	No	No
father-in-law	Yes	No	No	No

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

Other hyponyms of in-law include brother-in-law, daughter-in-law, mother-in-law, sister-in-law, son-in-law.

in-law (noun)	father-in-law (noun)
a relative by marriage	the father of your spouse