comparative degree vs furthest

comparative degree is a noun but furthest is not a noun.

Word	Noun	Adjective	Verb	Adverb
comparative degree	Yes	No	No	No
furthest	No	Yes	No	Yes

comparative degree (noun)	furthest (noun)
the comparative form of an adjective or adverb

comparative degree (adjective)	furthest (adjective)
	(comparatives of `far') most remote in space or time or order

comparative degree (adverb)	furthest (adverb)
	to the greatest distance in space or time (`farthest' is used more often than `furthest' in this physical sense)
	to the greatest degree or extent or most advanced stage (`furthest' is used more often than `farthest' in this abstract sense)