Extragonadal teratoma is an extremely rare, benign or malignant germ cell tumor characterized, clinically, by a teratoma presenting in an extragonadal location (e.g. retroperitoneum, mediastinum, craniofacial or sacrococcygeal region, intraosseous, solid organs) and, histologically, by displaying well-differentiated structures, as well as immature elements. Presenting symptoms are variable depending on size and location of tumor.

An organ or organ-system abnormality that consists of uncontrolled autonomous cell-proliferation which can occur in any part of the body as a benign or malignant neoplasm (tumour).