thrombocytopenia:

An abnormally low number of thrombocytes (platelets); the condition may result in abnormal bleeding and bruising. The normal platelet range is 150,000-300,000 per millimeter of blood. Thrombocytopenia may be controlled by the administration of certain cytokines or by removal of the spleen.

a condition in which there are too few platelets (cells produced in the bone marrow that are needed for clotting). 

Low platelet counts. Because platelets help to stop bleeding, severe thrombocytopenia needs to be recognized and treated accordingly with platelet transfusions.