A type of cancer that is usually slow growing and arises from special endocrine cells widely scattered throughout the body. These cells are most commonly found in the gastrointestinal system and sometimes in the lungs and other sites. Carcinoid tumors are potentially malignant and can, at times, spread to nearby lymph nodes, the liver, and elsewhere. They may secrete potent substances such as serotonin, prostaglandins, histamine, and other hormones which can affect the cardiovascular and digestive systems causing the carcinoid syndrome. Typical vs atypical carcinoids see below. The two types are distinguished from each other by their appearance under the microscope.
Typical carcinoid (TC): Typical carcinoids are nine times as common as atypical ones. Typical carcinoids grow slowly
Atypical carcinoid(AC): Atypical Carcinoid is faster growing than typical carcinoid. Atypical carcinoid (AC) is an intermediate form of tumor between low-grade malignant typical carcinoid (TC) and high-grade malignant small cell carcinoma (SCC), which represent the two ends of the spectrum of neuroendocrine tumors
Nonfunctioning carcinoids can be detected similarly to other space-occupying lesions, eg, by angiography, CT, or MRI, depending on the site. Small-bowel carcinoids may exhibit filling defects or other abnormalities on barium x-ray studies. Definitive diagnosis is made histologically
Functioning carcinoids are suspected on the basis of the symptoms and signs, and diagnosis is confirmed by demonstrating increased urinary excretion of the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA).

This definition is in context to Cancer. See more contextual defintions for Carcinoid (Cancer - Tumor).