A neoplasm arising from the epithelial cells of the thymus. Although thymomas are usually encapsulated tumors, they may invade the capsule and infiltrate the surrounding tissues or even metastasize to distant anatomic sites. The following morphologic subtypes are currently recognized: type A, type B, type AB, metaplastic, micronodular, microscopic, and sclerosing thymoma. Thymomas type B are further subdivided into types B1, B2, and B3. Thymoma type B3 usually has the most aggressive clinical course.

A neoplasm originating from thymic tissue, usually benign, and frequently encapsulated. Although it is occasionally invasive, metastases are extremely rare. It consists of any type of thymic epithelial cell as well as lymphocytes that are usually abundant. Malignant lymphomas that involve the thymus, e.g., lymphosarcoma, Hodgkin's disease (previously termed granulomatous thymoma), should not be regarded as thymoma. (From Stedman, 25th ed)

A tumor of the thymus, an organ that is part of the lymphatic system and is located in the chest, behind the breastbone.

A tumor originating from the epithelial cells of the thymus.

Thymoma originates within the epithelial cells of the thymus, a lymphoid organ located in the anterior mediastinum. This organ is located behind the sternum in front of the great vessels; it reaches its maximum weight at puberty and undergoes involution thereafter.