Teratoma

A teratoma is a type of tumour that derives from germ cells. Teratomas
usually start from cells in the testes in men, the ovaries in women and in
the sacrum in children. A teratoma is not a cancer.

Teratomas often contain well-differentiated cells which can result in
tissues growing in a teratoma which are quite different to the surrounding
tissue - ovarian teratomas have been known to grow hair and teeth.
Testicular teratomas are generally less well-differentiated, and have a
worse prognosis (chances of recovery are not as high).

A teratoma is also called a dermoid cyst.