What is the medical term for an excessive number of cells in the blood?

Polycythemia refers to an excessive number of red blood cells in the bloodstream, leading to increased blood viscosity and potential complications such as thrombosis. This condition can be caused by a variety of factors, including chronic hypoxia, which stimulates the production of erythropoietin, or it can occur as a primary disorder, as seen in polycythemia vera.

In contrast, leukemia involves the abnormal proliferation of white blood cells, often leading to a decrease in red blood cell production, resulting in anemia. Thrombocytopenia is characterized by a low platelet count, which can lead to increased bleeding and bruising, while aplastic anemia is a condition where the bone marrow fails to produce sufficient blood cells, leading to low counts of red blood cells, white blood cells, and platelets. Thus, polycythemia specifically describes the situation of having an excessive number of cells in the blood, accurately reflecting the essence of the question.