A rapidly progressing cancer that starts in blood-forming tissue, such as the bone marrow, and causes large numbers of white blood cells to be produced and enter the blood stream.
Leukemia is cancer of the blood cells; immature blood cells become cancer and they do not function the way they should. Instead, they crowd out healthy blood cells in the bone marrow.

Leukemia occurs most often in adults older than 55, but it is also the most common cancer in children younger than 15.
Different types of leukemia depend on the type of blood cell that becomes cancer.
It can be either acute or chronic. Acute leukemia is a fast-growing cancer that usually gets worse quickly. Chronic leukemia is a slower-growing cancer that gets worse slowly over time. The treatment and prognosis for leukemia depend on the type of blood cell affected and whether the leukemia is acute or chronic.

See reference for more information.

National Cancer Institute. NCI dictionary of cancer terms. Internet. Accessed on September 4, 2017.