The Collaborative International Dictionary of English v.0.48:

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
      (Brassica alba), black mustard (Brassica Nigra),
      wild mustard or charlock (Brassica Sinapistrum).
      [1913 Webster]

   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).
         [1913 Webster]

   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.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

black mustard
    n 1: widespread Eurasian annual plant cultivated for its pungent
         seeds; a principal source of table mustard [syn: black
         mustard, Brassica nigra]