underrating vs approximation

underrating and approximation both are nouns.

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

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

underrating: an estimation that is too low; an estimate that is less than the true or actual value
approximation: an approximate calculation of quantity or degree or worth

Other hypernyms of underrating include estimate, estimation, idea.

underrating (noun)	approximation (noun)
an estimation that is too low; an estimate that is less than the true or actual value	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