The term "Mature Teratomas" refers to a type of tumor made up of different types of tissue, including hair, teeth, and bone. The antonyms for mature teratomas can be described as tumors that are not fully developed or made up of only one type of tissue. These may include germ cell tumors, which are tumors that arise from cells that produce eggs or sperm, or sarcomas, which are tumors that arise from connective tissue like bone, cartilage, or muscle. Other antonyms may include benign tumors, which are not cancerous, or carcinoma, which is a cancerous tumor that arises from cells lining an organ or tissue.