a : a pungent yellow condiment consisting of the pulverized seeds of various mustard plants (such as Sinapis alba, Brassica juncea, and B. nigra) either dry or made into a paste or sauce (as by mixing with water or vinegar) and sometimes adulterated with other substances (such as turmeric) or mixed with spices
b : the seed of a mustard plant used as a spice and in medicine as a stimulant and diuretic, an emetic, or a counterirritant