schwannoma

n. abnormal tissue growth originating in the Schwann cells of the myelin sheath (Medicine)

A schwannoma is a kind of tumor originating from the Schwann cells. The insulating myelin sheath which covers peripheral nerves is produced by the Schwann cells. Hence, one kind of tumor originating from the Schwann cells is called a schwannoma. Schwannomas are very homogenous tumors consisting only of Schwann cells. The tumor cells always stay on the outside of the nerve, but the tumor itself may either push the nerve aside and/or up against a bony structure (thereby possibly causing damage). Schwannomas are relatively slow growing. For reasons not yet understood, schwannomas are mostly benign and less than 1 ecome malignant (degenerate into cancer).
See more at Wikipedia.org...

Schwannoma /schwan·no·ma/ a neoplasm originating from Schwann cells of neurons; schwannomas include neurofibromas and neurilemomas. [more]Schwannoma - Community and Resources

[shwah-NO-ma]
A tumor of the peripheral nervous system that arises in the nerve sheath (protective covering). It is almost always benign, but rare malignant schwannomas have been reported.