also called carcinoids. Tumors that develop from neuroendocrine (nerve and endocrine) cells, usually in the digestive tract or lung. The cells from these tumors release certain hormones into the bloodstream. In a small percentage of people, the hormone levels get high enough to cause facial flushing, wheezing, diarrhea, a fast heartbeat, and other symptoms. See also carcinoid syndrome, endocrine glands, hormone.