A tumor (also known as a neoplasm) is an abnormal tissue growth. Neoplasm means new formation. Tumors can be either malignant (cancerous) or nonmalignant (benign), but either type may require therapy to remove or reduce its size. In either case, the tumor's growth is unregulated by normal genetic and somatic body control mechanisms. Usually the growth is not beneficial to the organ in which it is developing. Mutation in normal functioning genes may lead to the expression of oncogenes and affect expression of tumor suppressor genes to produce cancerous tumors.

Normally, cells are generated at a rate needed to replace those that die or are needed for an individual's growth and development. Moreover, cells become differentiated into specialized cell forms (muscle cells, bone cells). Genetic controls modulate the formation of any given cells. The process of some cells becoming muscle cells, some becoming nerve cells, and so on...