1. (Science: botany) 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).
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.
Origin: OF. Moustarde, F. Moutarde, fr. L. Mustum must, mustard was prepared for use by being mixed with must. See Must.