approximate vs guess

approximate is not a noun while guess is a noun.

Word	Noun	Adjective	Verb	Adverb
approximate	No	Yes	Yes	No
guess	Yes	No	Yes	No

As verbs, approximate and guess are synonyms defined as:

approximate and guess: judge tentatively or form an estimate of (quantities or time)

Other synonyms of approximate include estimate, gauge, judge.

approximate (noun)	guess (noun)
	an estimate based on little or no information
	a message expressing an opinion based on incomplete evidence

approximate (verb)	guess (verb)
judge tentatively or form an estimate of (quantities or time)	expect, believe, or suppose
be close or similar	judge tentatively or form an estimate of (quantities or time)
	guess correctly; solve by guessing
	put forward, of a guess, in spite of possible refutation