Multiple myeloma: A bone marrow cancer that involves a type of white blood cell called a plasma (or myeloma) cell. The tumor cells in myeloma can form a single collection (plasmacytoma) or many tumors (multiple myeloma). Plasma cells are normally part of the immune system; they make antibodies. Because myeloma patients have an excess of identical plasma cells, they have too much of one type of antibody. As myeloma cells increase in number, they damage and weaken the bones, causing pain and often fractures. When bones are damaged, too much calcium is released into the blood, leading to loss of appetite, nausea, thirst, fatigue, muscle weakness, restlessness, and confusion. Myeloma cells prevent the bone marrow from forming normal plasma cells and other white blood cells that are important to the immune system, so patients with multiple myeloma may not be able to fight infections. Myeloma cells can also prevent the growth of new red blood cells in the marrow, causing anemia. Excess antibody proteins and calcium may prevent the kidneys from filtering and cleaning the blood properly. Chemotherapy and bone marrow transplant are the primary treatments. Also known as plasma cell myeloma and myeloma.