lilac vs Japanese lilac

lilac and Japanese lilac both are nouns.

Word	Noun	Adjective	Verb	Adverb
lilac	Yes	Yes	No	No
Japanese lilac	Yes	No	No	No

As nouns, Japanese lilac is a hyponym of lilac; that is, Japanese lilac is a word with a more specific, narrower meaning than lilac:

lilac: any of various plants of the genus Syringa having large panicles of usually fragrant flowers
Japanese lilac: lilac of northern China having ovate leaves and profuse early summer rose-lilac flowers

lilac (noun)	Japanese lilac (noun)
any of various plants of the genus Syringa having large panicles of usually fragrant flowers	lilac of northern China having ovate leaves and profuse early summer rose-lilac flowers

lilac (adjective)	Japanese lilac (adjective)
of a pale purple color