compassion vs mercy

compassion and mercy both are nouns.

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

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

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

Other hypernyms of compassion include mercifulness.

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

Other hyponyms of compassion include heartstrings, mellowness, tenderheartedness, tenderness, mercifulness.

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