<haematology> Increase in the haemoglobin content of the blood, either because of a reduction in plasma volume or an increase in red cell numbers. The latter may be a result of abnormal proliferation of red cell precursors (polycythaemia vera, Vaquez Osler disease). ... Origin: Gr. Haima = blood ... (18 Nov 1997) ... 
increase in the haemaglobin concentration of the blood
An excess of red cells in the blood.
polycythaemia is a condition marked by an abnormal increase in the number of circulating red blood cells, leading to a thickening of the blood.
an increased amount of red blood cells in the blood