Dictionary.net

The occurrence of myeloid tissue in extramedullary sites. Specifically it is a syndrome characterized by splenomegaly, anemia, the presence of nucleated erythrocytes and immature granulocytes in the circulating blood, and extramedullary hemoatopoiesis in the liver and spleen. The primary form is also known as agnogenic myeloid metaplasia, myelosclerosis, and myelofibrosis. The secondary or symptomatic form may be associated with various diseases, including carcinomatosis, tuberculosis, leukemia, and polycythemia vera. (Dorland, 27th ed)