Medical Definition of Hematology. Hematology: The diagnosis, treatment, and prevention of diseases of the blood and bone marrow as well as of the immunologic, hemostatic (blood clotting) and vascular systems. Because of the nature of blood, the science of hematology profoundly affects the understanding of many diseases Hematology, also spelled haematology, is the branch of medicine concerned with the study of the cause, prognosis, treatment, and prevention of diseases related to blood. ...