overestimate vs approximation

overestimate and approximation both are nouns.

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

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

Other hypernyms of overestimate include estimate, estimation, idea.

overestimate (noun)	approximation (noun)
a calculation that results in an estimate that is too high	the act of bringing near or bringing together especially the cut edges of tissue
an appraisal that is too high	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

overestimate (verb)	approximation (verb)
make too high an estimate of
assign too high a value to