approximate vs cipher

approximate is not a noun while cipher is a noun.

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

As verbs, cipher is a hypernym of approximate; that is, cipher is a word with a broader meaning than approximate:

Other hypernyms of approximate include calculate, compute, cypher, figure, reckon, work out.

approximate (noun)	cipher (noun)
	a secret method of writing
	a message written in a secret code
	a person of no influence
	a quantity of no importance; thing (object:), singular, negative pronoun; pronoun, thing, singular; quantifier: negative existential
	a mathematical element that when added to another number yields the same number

approximate (verb)	cipher (verb)
judge tentatively or form an estimate of (quantities or time)	make a mathematical calculation or computation
be close or similar	convert ordinary language into code