countersink vs counterbore

countersink and counterbore both are nouns.

Word	Noun	Adjective	Verb	Adverb
countersink	Yes	No	Yes	No
counterbore	Yes	No	No	No

As nouns, countersink and counterbore are synonyms defined as:

countersink and counterbore: a bit for enlarging the upper part of a hole

Other synonyms of countersink include countersink bit.

countersink (noun)	counterbore (noun)
a bit for enlarging the upper part of a hole	a bit for enlarging the upper part of a hole
a hole (usually in wood) with the top part enlarged so that a screw or bolt will fit into it and lie below the surface

countersink (verb)	counterbore (verb)
insert (a nail or screw below the surface, as into a countersink)