ovarian teratoma (Monarch Disease Ontology id MONDO_0005602) is a non-seminomatous germ cell tumor arising from the ovary. It is characterized by the presence of various tissues which correspond to the different germinal layers (endoderm, mesoderm, and ectoderm). According to the level of differentiation of the tissues which comprise the tumor, ovarian 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. 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. Also known as: germ cell teratoma of ovary, germ cell teratoma of the ovary, ovarian germ cell teratoma, teratoma of ovary, teratoma of the ovary.