a chronic blood disorder in which the bone marrow makes too many red blood cells, although it can also make too many white blood cells and platelets. Blood clots can become a problem because of high platelet counts and “thickened” blood. Often, the spleen is enlarged. For some people, the bone marrow is displaced by fibrous tissue (myelofibrosis). PV is not a true cancer, but rarely it evolves into acute myelogenous leukemia (AML). See also bone marrow, leukemia, platelet, red blood cells, spleen, white blood cells.