Carcinoid is a slow-growing type of neuroendocrine tumor, originating in the cells of the neuroendocrine system.

In 2000, the World Health Organization

World Health Organization

The World Health Organization is a specialized agency of the United Nations that acts as a coordinating authority on international public health. Established on 7 April 1948, with headquarters in Geneva, Switzerland, the agency inherited the mandate and resources of its predecessor, the Health...

 redefined "carcinoid", but this new definition has not been accepted by all practitioners. This has led to some complexity in distinguishing between carcinoid and other neuroendocrine tumors in the literature. According to the American Cancer Society, the 2000 WHO definition states:

The WHO now divides these growths into neuroendocrine tumors and neuroendocrine cancers.