Source : Webster's Revised Unabridged Dictionary (1913)

Mustard \Mus"tard\, n. [OF. moustarde, F. moutarde, fr. L.
   mustum must, -- mustard was prepared for use by being mixed
   with must. See {Must}, n.]
   1. (Bot.) The name of several cruciferous plants of the genus
      {Brassica} (formerly {Sinapis}), as white mustard ({B.
      alba}), black mustard ({B. Nigra}), wild mustard or
      charlock ({B. Sinapistrum}).

   Note: There are also many herbs of the same family which are
         called mustard, and have more or less of the flavor of
         the true mustard; as, bowyer's mustard ({Lepidium
         ruderale}); hedge mustard ({Sisymbrium officinale});
         Mithridate mustard ({Thlaspi arvense}); tower mustard
         ({Arabis perfoliata}); treacle mustard ({Erysimum
         cheiranthoides}).

   2. A powder or a paste made from the seeds of black or white
      mustard, used as a condiment and a rubefacient. Taken
      internally it is stimulant and diuretic, and in large
      doses is emetic.

   {Mustard oil} (Chem.), a substance obtained from mustard, as
      a transparent, volatile and intensely pungent oil. The
      name is also extended to a number of analogous compounds
      produced either naturally or artificially.

Source : WordNet®

Erysimum cheiranthoides
     n : slender yellow-flowered European mustard often troublesome
         as a weed; formerly used as an anthelmintic [syn: {wormseed
         mustard}]