n. A plant of the genus Brassica, formerly classed as Sinapis.n. The seed of mustard crushed and sifted (and often adulterated), used in the form of a paste as a condiment. or, in the form of a poultice (sinapism), plaster, or prepared paper (mustard-paper), as a rubefacient.n. One of numerous mustard-like plants, almost all cruciferous: used with a qualifying word. See names belown. Clypeola Jonthlaspi.n. Sometimes, erroneously, the pennycress