burden vs millstone

burden and millstone both are nouns.

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

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

Other hyponyms of burden include burthen, dead load, live load, superload, overburden, overload, overload.

burden (noun)	millstone (noun)
weight to be borne or conveyed	one of a pair of heavy flat disk-shaped stones that are rotated against one another to grind the grain
the central idea that is expanded in a document or discourse	any load that is difficult to carry
an onerous or difficult concern	(figurative) something that hinders or handicaps
the central meaning or theme of a speech or literary work

burden (verb)	millstone (verb)
impose a task upon, assign a responsibility to
weight down with a load