ITP: Idiopathic thrombocytopenic purpura. A condition characterized by the sudden, abnormal lowering of the platelet count. The cause is unknown, but the immune system seems to play a major role in the process by eliminating the platelets. ITP can lead to dangerously low platelet counts that can be associated with spontaneous bleeding. Purpura are purplish bruises under the skin that are often seen in this condition. ITP requires treatments directed at suppressing the immune system.