n 1  :  a pungent yellow condiment consisting of the pulverized seeds of the black mustard or sometimes the white mustard either dry or made into a paste and serving as a stimulant and diuretic or in large doses as an emetic and as a counterirritant when applied to the skin as a poultice  2  :  any of several herbs (genus Brassica of the family Cruciferae, the mustard family) with lobed leaves, yellow flowers, and linear beaked pods - see BLACK MUSTARD1 WHITE MUSTARD   3 a  :  MUSTARD GAS  b  :  NITROGEN MUSTARD .
 
 
 
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