blastoma A tumor thought to arise in embryonic tissue. This term is commonly used as part of the name for a tumor, as in glioblastoma and medulloblastoma (types of brain tumors), hepatoblastoma (a liver tumor), nephroblastoma (a Wilms tumor of the kidney), neuroblastoma (a childhood tumor of neural origin), osteoblastoma (a bone tumor), and retinoblastoma (a tumor of the retina in the eye).