A rare myeloproliferative neoplasm characterized by stem-cell derived clonal over proliferation of mature myeloid lineages, such as erythrocytes, leukocytes, and megakaryocytes, with variable degrees of megakaryocyte atypia, associated with reticulin and/or collagen bone marrow fibrosis, osteosclerosis, ineffective erythropoiesis, angiogenesis, extramedullary hematopoiesis, and abnormal cytokine expression.

A hemangioma is a benign tumor characterized by blood-filled spaces lined by benign endothelial cells. A hemangioma characterized by large endothelial spaces (caverns) is called a cavernous hemangioma (in contrast to a hemangioma with small endothelial spaces, which is called capillary hemangioma).