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 ]

☞ 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 ]