bowling pin vs ninepin

bowling pin and ninepin both are nouns.

Word	Noun	Adjective	Verb	Adverb
bowling pin	Yes	No	No	No
ninepin	Yes	No	No	No

As nouns, ninepin is a hyponym of bowling pin; that is, ninepin is a word with a more specific, narrower meaning than bowling pin:

bowling pin: a club-shaped wooden object used in bowling; set up in triangular groups of ten as the target
ninepin: a bowling pin of the type used in playing ninepins or (in England) skittles

Other hyponyms of bowling pin include candlepin, duckpin, headpin, kingpin, skittle, skittle pin, tenpin.

bowling pin (noun)	ninepin (noun)
a club-shaped wooden object used in bowling; set up in triangular groups of ten as the target	a bowling pin of the type used in playing ninepins or (in England) skittles