A neoplasm with neuroendocrine differentiation that arises from the appendix. It includes well differentiated neuroendocrine tumors (low and intermediate grade) and poorly differentiated neuroendocrine carcinomas (high grade). The disease is appendix neuroendocrine neoplasm (MONDO_0024501, a Monarch Disease Ontology term). Also known as: neuroendocrine neoplasm of vermiform appendix, vermiform appendix NET, vermiform appendix neuroendocrine neoplasm, vermiform appendix neuroendocrine tumor, vermiform appendix neuroendocrine tumor, well differentiated, low or intermediate grade, vermiform appendix neuroendocrine tumour.