Definition of islet cell tumor: A tumor that forms in islet cells (hormone-making cells) of the pancreas. Islet cell tumors may be benign (not cancer) or malignant (cancer). Islet cells make several different hormones that affect body functions, including controlling the amount of glucose (sugar) in the blood and helping digest food in the stomach. Functional islet cell tumors make extra amounts of these hormones, which can cause symptoms. Nonfunctional islet cell tumors do not make extra amounts of hormones, but they may cause symptoms as they grow and spread. Also called pancreatic endocrine tumor and pancreatic neuroendocrine tumor.