thymoma definition
Freebase (0.00 / 0 votes) Rate this definition: Thymoma. Thymoma is a tumor originating from the epithelial cells of the thymus. Thymoma is an uncommon tumor, best known for its association with the neuromuscular disorder myasthenia gravis. Thymoma is found in 15% of patients with myasthenia gravis. Once diagnosed, thymomas may be removed surgically.