guessing vs approximation

guessing and approximation both are nouns.

Word	Noun	Adjective	Verb	Adverb
guessing	Yes	No	No	No
approximation	Yes	No	No	No

As nouns, approximation is a hypernym of guessing; that is, approximation is a word with a broader meaning than guessing:

Other hypernyms of guessing include estimate, estimation, idea.

guessing (noun)	approximation (noun)
an estimate based on little or no information	the act of bringing near or bringing together especially the cut edges of tissue
	the quality of coming near to identity (especially close in quantity)
	an approximate calculation of quantity or degree or worth
	an imprecise or incomplete account