teratoma (MONDO_0002601, a Monarch Disease Ontology term) is a non-seminomatous germ cell tumor characterized by the presence of various tissues which correspond to the different germinal layers (endoderm, mesoderm, and ectoderm). It occurs in the testis, ovary, and extragonadal sites including central nervous system, mediastinum, lung, and stomach. According to the level of differentiation of the tissues which comprise the tumor, teratomas are classified as mature or immature. Mature teratomas are composed of well differentiated, adult-type tissues. Immature teratomas are composed of immature, fetal-type tissues. Testicular teratomas in children follow a benign clinical course. Mature ovarian teratomas without a fetal-type component have an excellent outcome. The prognosis of immature ovarian teratomas is related to the grade and stage of the tumor.