millstone vs burden

millstone and burden both are nouns.

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

As nouns, burden is a hypernym of millstone; that is, burden is a word with a broader meaning than millstone:

Other hypernyms of millstone include load, loading.

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

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