plea vs insanity plea

plea and insanity plea both are nouns.

Word	Noun	Adjective	Verb	Adverb
plea	Yes	No	No	No
insanity plea	Yes	No	No	No

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

plea: (law) a defendant's answer by a factual matter (as distinguished from a demurrer)
insanity plea: (criminal law) a plea in which the defendant claims innocence due to mental incompetence at the time

Other hyponyms of plea include counterplea, dilatory plea, plea of insanity.

plea (noun)	insanity plea (noun)
an answer indicating why a suit should be dismissed	(criminal law) a plea in which the defendant claims innocence due to mental incompetence at the time
a humble request for help from someone in authority
(law) a defendant's answer by a factual matter (as distinguished from a demurrer)