A progressive, malignant neoplasm of the blood-forming organs, marked by diffuse replacement of the bone marrow development of leukocytes and their precursors in the blood and bone marrow. It is accompanied by a reduced number of erythrocytes and blood platelets, resulting in anemia and increased susceptibility to infection and hemorrhage. Other typical symptoms include fever, pain in the joints and bones, and swelling of the lymph nodes, spleen, and liver.

Leukemia is classified clinically in several ways:
- acute versus chronic, terms that have become altered from their usual meanings and refer to the degree of cell differentiation;
- the predominant proliferating cells: myelocytic, granulocytic, or lymphocytic; and
- increase in or maintenance of the number of abnormal cells in the blood — preleukemic.

Adapted from The free medical dictionary by Farlex. Leukemia. Internet. Accessed on 22 April, 2013.