A true neoplasm composed of a number of different types of tissue, none of which is native to the area in which it occurs. It is composed of tissues that are derived from three germinal layers, the endoderm, mesoderm, and ectoderm. They may be solid or cystic and are classified histologically as mature, immature, and malignant. (From Dorland, 27th ed & DeVita Jr et al., Cancer: Principles & Practice of Oncology, 3d ed, p1642).