mercy vs compassion

mercy and compassion both are nouns.

Word	Noun	Adjective	Verb	Adverb
mercy	Yes	No	No	No
compassion	Yes	No	No	No

As nouns, compassion is a hyponym of mercy; that is, compassion is a word with a more specific, narrower meaning than mercy:

mercy: a disposition to be kind and forgiving
compassion: the humane quality of understanding the suffering of others and wanting to do something about it

Other hyponyms of mercy include pity, forgivingness, kindness, lenience, leniency, lenity, mildness.

As nouns, compassion is a hypernym of mercy; that is, compassion is a word with a broader meaning than mercy:

Other hypernyms of mercy include compassionateness.

mercy (noun)	compassion (noun)
leniency and compassion shown toward offenders by a person or agency charged with administering justice	the humane quality of understanding the suffering of others and wanting to do something about it
alleviation of distress; showing great kindness toward the distressed	a deep awareness of and sympathy for another's suffering
a disposition to be kind and forgiving
the feeling that motivates compassion
something for which to be thankful