Mustard plaster: A mild mixture of mustard seed powder and flour with water or egg white, and spread on a moist cloth that is then applied to the chest, abdomen or back. The mustard mixture should never come in contact with the skin as it will lead to burning and blistering. It is used for many ailments, including the flu, coughs, and colds. It acts as a counterirritant and increases the local blood supply. a mustard plaster is a form of poultice. See also: Cataplasm.