immature

Definition of immature teratoma
National Cancer Institute

immature teratoma

(IH-muh-CHOOR TAYR-uh-TOH-muh)
A type of germ cell tumor that is often made up of several different types of tissue, such as hair, muscle, and bone. Immature teratomas have cells that look very different from normal cells under a microscope. They are usually malignant (cancer) and may spread to other parts of the body. Some immature teratomas make enzymes or hormones that may cause signs and symptoms of disease. Immature teratomas usually occur in the sacrum or tailbone in newborns or in the ovaries of girls at the start of puberty.

Source: NCI Dictionary of Cancer Terms