impediment vs millstone

impediment and millstone both are nouns.

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

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

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

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

impediment (noun)	millstone (noun)
any structure that makes progress difficult	one of a pair of heavy flat disk-shaped stones that are rotated against one another to grind the grain
something immaterial that interferes with or delays action or progress	any load that is difficult to carry
	(figurative) something that hinders or handicaps