oil-producing plant spice or flavor-producing plant leafy vegetable plant mustard plant Mustards are several plant species in the genera *Brassica* and *Sinapis* whose small mustard seeds are used as a spice and, by grinding and mixing them with water, vinegar or other liquids, are turned into the condiment known as mustard or prepared mustard. The seeds are also pressed to make mustard oil, and the edible leaves can be eaten as mustard greens. brassica species