wig vs postiche

wig and postiche both are nouns.

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

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

wig: hairpiece covering the head and made of real or synthetic hair
postiche: a covering or bunch of human or artificial hair used for disguise or adornment

Other hypernyms of wig include false hair, hairpiece.

wig (noun)	postiche (noun)
hairpiece covering the head and made of real or synthetic hair	something that is a counterfeit; not what it seems to be
British slang for a scolding	a covering or bunch of human or artificial hair used for disguise or adornment