dowel pin vs holdfast

dowel pin and holdfast both are nouns.

Word	Noun	Adjective	Verb	Adverb
dowel pin	Yes	No	No	No
holdfast	Yes	No	No	No

As nouns, holdfast is a hypernym of dowel pin; that is, holdfast is a word with a broader meaning than dowel pin:

dowel pin: a fastener that is inserted into holes in two adjacent pieces and holds them together
holdfast: restraint that attaches to something or holds something in place

Other hypernyms of dowel pin include fastener, fastening, fixing.

dowel pin (noun)	holdfast (noun)
a fastener that is inserted into holes in two adjacent pieces and holds them together	restraint that attaches to something or holds something in place