iniquity vs foul play

iniquity and foul play both are nouns.

Word	Noun	Adjective	Verb	Adverb
iniquity	Yes	No	No	No
foul play	Yes	No	No	No

As nouns, foul play is a hyponym of iniquity; that is, foul play is a word with a more specific, narrower meaning than iniquity:

Other hyponyms of iniquity include devilry, deviltry, irreverence, violation, sexual immorality.

iniquity (noun)	foul play (noun)
morally objectionable behavior	unfair or dishonest behavior (especially involving violence)
an unjust act