特发性血小板减少性紫癜（Idiopathic thrombocytopenic purpura，ITP）是因血小板免疫性破坏，导致外周血中血小板减少的出血性疾病，以广泛皮肤、粘膜及内脏出血、血小板减少、骨髓巨核细胞发育成熟障碍、血小板生存时间缩短及抗血小板自身抗体出现等为特征，是最常见的血小板减少性紫癜，通常认为是一种自体免疫性疾病，也称为免疫性血小板减少症(immune thrombocytopenia)、自身免疫性血小板减少性紫癜（Autoimmune thrombocytopenia，ATP)。

Immune thrombocytopenia (ITP), also known as immune thrombocytopenic purpura, idiopathic immune thrombocytopenia, primary immune thrombocytopenia, Idiopathic thrombocytopenic purpura, primary immune thrombocytopenic purpura, or autoimmune thrombocytopenic purpura, is defined as isolated low platelet count (thrombocytopenia) with normal bone marrow and the absence of other causes of thrombocytopenia. It causes a characteristic purpuric rash and an increased tendency to bleed. Two distinct clinical syndromes manifest as an acute condition in children and a chronic condition in adults. The acute form often follows an infection and has a spontaneous resolution within two months. Chronic immune thrombocytopenia persists longer than six months with a specific cause being unknown.