Thymoma: a rare type of tumor that is the most common tumor type located in the area in the center of the chest between the lungs (the anterior mediastinum). Thymomas originate from epithelial cells in the thymus, a gland located behind the breast bone in the mediastinum, which is responsible for the development of many immunologic functions in early life. The thymus contains both lymphoid (immune) cells and lining (epithelial) cells and begins to reduce in size after an individual reaches puberty. The cells of thymomas appear similar to normal thymus cells, while cancers of the thymus have abnormal, cancerous-appearing cells. Cancers of the thymus (thymic carcinomas) have a much greater tendency to spread to other areas of the body than thymomas. Myasthenia gravis and also other autoimmune diseases have been associated with thymoma.