MALT lymphoma: A low grade type of malignancy that arises in cells in mucosal tissue which are involved in antibody production. These lymphomas occur most often in the stomach but can also arise in the lung, thyroid, salivary glands, eye, skin, or soft tissues. MALT stands for mucosa-associated lymphoid tissue. MALT lymphomas are typically slow growing and are usually diagnosed at an early stage. They may be treated with low doses of radiotherapy or removed by surgery. If they have spread, they are treated with chemotherapy. The outlook is good, even when the disease is quite widespread.