- either of a pair of large, flat, round stones between which grain or other substances are ground
- stone used for these, usually a hard sandstone or conglomerate
- a heavy burden
- something that grinds, pulverizes, or crushes
- One of a pair of cylindrical stones used in a mill for grinding grain.
- a. A source of worry or distress: a responsibility that became a millstone.b. An obstacle to success. See Synonyms at burden.