1. A malignant neoplasm arising from vascular endothelial cells; the term may be used generally or may denote a specific histologic subtype, usually hemangiosarcoma but also others; such as, lymphangiosarcoma or hemangiopericytoma.
2. A rare malignant tumor of vascular origin, formed by proliferation of endothelial tissue lining irregular vascular channels; it usually occurs in the skin, soft tissues, breast, or liver.