hairpin vs bobby pin

hairpin and bobby pin both are nouns.

Word	Noun	Adjective	Verb	Adverb
hairpin	Yes	No	No	No
bobby pin	Yes	No	No	No

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

hairpin: a double pronged pin used to hold women's hair in place
bobby pin: a flat wire hairpin whose prongs press tightly together; used to hold bobbed hair in place

Other hyponyms of hairpin include grip, hairgrip, bodkin.

hairpin (noun)	bobby pin (noun)
a double pronged pin used to hold women's hair in place	a flat wire hairpin whose prongs press tightly together; used to hold bobbed hair in place