postiche vs imitation

postiche and imitation both are nouns.

Word	Noun	Adjective	Verb	Adverb
postiche	Yes	No	No	No
imitation	Yes	Yes	No	No

As nouns, imitation is a hypernym of postiche; that is, imitation is a word with a broader meaning than postiche:

postiche (noun)	imitation (noun)
something that is a counterfeit; not what it seems to be	copying (or trying to copy) the actions of someone else
a covering or bunch of human or artificial hair used for disguise or adornment	something copied or derived from an original
	the doctrine that representations of nature or human behavior should be accurate imitations
	a representation of a person that is exaggerated for comic effect

postiche (adjective)	imitation (adjective)
	not genuine or real; being an imitation of the genuine article