approximate vs quantise

approximate is an adjective but quantise is not an adjective.

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

As verbs, quantise is a hyponym of approximate; that is, quantise is a word with a more specific, narrower meaning than approximate:

approximate: judge tentatively or form an estimate of (quantities or time)
quantise: approximate (a signal varying continuously in amplitude) by one whose amplitude is restricted to a prescribed set of discrete values

Other hyponyms of approximate include quantize, misgauge, place, put, set, give, lowball, underestimate, assess, make, count, reckon, truncate, guesstimate.

approximate (verb)	quantise (verb)
judge tentatively or form an estimate of (quantities or time)	apply quantum theory to; restrict the number of possible values of (a quantity) or states of (a physical entity or system) so that certain variables can assume only certain discrete magnitudes that are integral multiples of a common factor
be close or similar	approximate (a signal varying continuously in amplitude) by one whose amplitude is restricted to a prescribed set of discrete values