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):

mustard
    n 1: any of several cruciferous plants of the genus Brassica
    2: pungent powder or paste prepared from ground mustard seeds
       [syn: mustard, table mustard]
    3: leaves eaten as cooked greens [syn: mustard, mustard
       greens, leaf mustard, Indian mustard]

Easton's 1897 Bible Dictionary:

Mustard
   a plant of the genus sinapis, a pod-bearing, shrub-like plant,
   growing wild, and also cultivated in gardens. The little round
   seeds were an emblem of any small insignificant object. It is
   not mentioned in the Old Testament; and in each of the three
   instances of its occurrence in the New Testament (Matt. 13:31,
   32; Mark 4:31, 32; Luke 13:18, 19) it is spoken of only with
   reference to the smallness of its seed. The common mustard of
   Palestine is the Sinapis nigra. This garden herb sometimes grows
   to a considerable height, so as to be spoken of as "a tree" as
   compared with garden herbs.