A circumscribed neoplasm arising from the glandular epithelium of the appendix. Morphologically, it is characterized by a proliferation of neoplastic glandular cells and it is associated with dysplasia. According to the growth pattern, it may be classified as tubular, villous, or tubulovillous. The dysplasia is classified as mild, moderate, or severe. The disease is appendix adenoma (MONDO_0006088, a Monarch Disease Ontology entry). Also known as: vermiform appendix adenoma.