Cancer is the disease of the cells. It is an abnormal growth of cells, which tend to reproduce in an uncontrolled way and, in some cases, spread or metastasize. A cancerous growth or tumor is also known as a malignant growth or tumor. A growth or tumor, which is non-malignant is called benign. Such tumors are not cancer.

Cancer is not a single disease. It is a group of more than hundred different and distinctive diseases. It is not contagious. Cancer can involve any tissue of the body and have many different forms in each body area. Most cancers are named for the type of cell or organ in which they start. If a cancer spreads (metastasizes), the new tumor bears the same name as the original(primary) tumor.

Cancer is the Latin word for crab. The ancients used the word to mean a malignancy, doubtless because of the crab-like tenacity a malignant tumor sometimes seems to show in grasping the tissues it invades. Cancer may also be called malignancy, a malignant tumor, or a neoplasm (literally, a new growth).

Cancer results from mutations of certain genes that allow the cells to begin their uncontrolled growth. These mutations are either inherited or acquired. Acquired mutations are caused by repeated insults from triggers (e.g., cigarette smoke or ultraviolet rays) referred to as carcinogens. There is usually a latency period of years or decades between exposure to a carcinogen and the appearance of cancer. This, combined with the individual nature of susceptibility to cancer, makes it very difficult to establish a cause for many cancers.