A teratoma (from the Greek word teras, which means "monster") is a rare congenital tumor that emerges from embryonic germ cells. These tumors are most often found in the ovaries and testes, though they can appear anywhere in the body.
These tumors, which can be benign or malignant, contain cells and/or differentiated tissues that should not be found in the organ they afflict.
For instance, a cancerous ovarian teratoma might contain skin cells, cartilage, and lung tissue; a cystic benign ovarian teratoma (also called a dermoid cyst) is made up of epithelial tissues and often contains hair and teeth. In Stephen King's novel The Dark Half, the "absorbed twin" that was removed from the protagonist's brain would most likely be classified as a fetus in fetu rather than a teratoma in real life.
These tumors are most often found in newborns, children and adolescents, although they may develop later in life. Some people chronically develop these tumors. I knew one woman who had to have several of them removed from various parts of her body over the years; when I first met her, she'd recently had a tumor removed from her arm that she described as looking very much like a curled-up embryo.