Embryonal carcinoma: A malignant germ cell tumor that occurs most often in the testes and accounts for about 40% of testicular tumors. Under the microscope, these tumors may resemble tissues of early embryos. This type of tumor can grow rapidly and spread outside the testicle. Embryonal carcinoma of the ovary is rare. It usually occurs before the age of 30 and is often prepubertal.