embryonic tumor a neoplasm, usually malignant, which arises during intrauterine or early postnatal development from an organ rudiment or immature tissue; it forms immature structures characteristic of the part from which it arises, and may form other tissues as well. The term includes neuroblastoma and Wilms tumor, and is also used to include certain neoplasms presenting in later life, this usage being based on the belief that such tumors arise from embryonic rests.teratoma. SYN: embryoma.