Mustar is a thick yellowish-brown paste with a sharp taste made from the ground seeds of a mustard plant (white or yellow mustard, Sinapis hirta; brown or Indian mustard, Brassica juncea; or black mustard, Brassica nigra). The mustard seeds are mixed with water, vinegar or other liquids, as well as other flavorings and spices. A strong mustard can cause the eyes to water, burn the palate and inflame the nasal passages.