balk vs millstone

balk and millstone both are nouns.

Word	Noun	Adjective	Verb	Adverb
balk	Yes	No	Yes	No
millstone	Yes	No	No	No

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

balk: something immaterial that interferes with or delays action or progress
millstone: (figurative) something that hinders or handicaps

Other hyponyms of balk include albatross, bind, diriment impediment, drag, obstacle, obstruction, straitjacket.

balk (noun)	millstone (noun)
an illegal pitching motion while runners are on base	one of a pair of heavy flat disk-shaped stones that are rotated against one another to grind the grain
one of several parallel sloping beams that support a roof	any load that is difficult to carry
something immaterial that interferes with or delays action or progress	(figurative) something that hinders or handicaps
the area on a billiard table behind the balkline

balk (verb)	millstone (verb)
refuse to comply