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 (Matthew 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.

Noah Webster's New International Dictionary of the English Language

1. (n.) 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).

2. (n.) 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.