thrombocytopeniaPronunciation: /ˌθrɒmbə(ʊ)ˌsʌɪtə(ʊ)ˈpiːnɪə/Definition of thrombocytopenia noun [mass noun] Medicine deficiency of platelets in the blood. This causes bleeding into the tissues, bruising, and slow blood clotting after injury. Origin: 1920s: from thrombocyte + Greek penia 'poverty'thrombocytopenia in other Oxford dictionaries Definition of thrombocytopenia in the US English dictionary