Source : Webster's Revised Unabridged Dictionary (1913)

White mustard \White mustard\
   A kind of mustard ({Sinapis alba}) with rough-hairy foliage,
   a long-beaked hispid pod, and pale seeds, which yield mustard
   and mustard oil. The plant is also grown for forage.

Source : WordNet®

white mustard
     n : Eurasian mustard cultivated for its pungent seeds; a source
         of table mustard and mustard oil [syn: {Brassica hirta},
         {Sinapis alba}]