A neoplasm arising from the epithelium. It may be encapsulated or non-encapsulated but non-invasive. The neoplastic epithelial cells may or may not display cellular atypia or dysplasia. In the gastrointestinal tract, when dysplasia becomes severe it is sometimes called carcinoma in situ. Representative examples are pituitary gland adenoma, follicular adenoma of the thyroid gland, and adenomas (or adenomatous polyps) of the gastrointestinal tract. The disease is adenoma (MONDO_0004972). Also known as: acinic cell adenoma, adenomas.