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teratoma (te-ră-toh-mă) n. a tumour composed of a number of tissues that are not usually found at that site and are derived from all three germ layers. Teratomas most frequently occur in the testis and ovary, possibly derived from remnants of embryonic cells, and are often malignant. malignant t. of the testis a teratoma occurring in young men, frequently as a painless swelling of one testis. Treatment is by orchidectomy.