Cancer is a class of diseases characterized by uncontrolled division of cells and the ability of these cells to invade other tissues, either by direct growth into adjacent tissue (invasion) or by implantation into distant sites (metastasis). This unregulated growth is caused by damage to DNA, resulting in mutations to genes that control cell division. Several mutations are required to transform a normal cell into a malignant cell. These mutations are often caused by chemicals or physical agents called carcinogens, the best known being tobacco smoke. Some mutations can be inherited.

The word cancer is not used in medicine as it lacks precision, but remains the common name for this group of diseases in most languages. Instead, scientists and physicians use the word neoplasm (Latin neoplasia, new form). Tumor ("swelling" in Latin) is a more general term that describes an abnormal mass. In medicine, tumor is not synonymous with neoplasm; most tumors are not neoplasm, but rather inflammation surrounding an infection. Furthermore, many neoplasms do not cause a tumor, e.g. leukemia and myeloma. In the common language, however, tumor is synonymous with neoplasm. Tumors (and therefore neoplasms) can be malignant or benign. Therefore, cancer is synonymous with malignant tumor or, in medical language, malignant neoplasm.

Cancer can cause many different symptoms, depending on the site and character of the malignancy and whether there is metastasis. Usually, cancer is painless at an early stage. A definitive diagnosis usually requires the histologic examination of tissue by a specialized physician called a pathologist. This tissue is obtained by biopsy or surgery. Once diagnosed, cancer is usually treated with surgery, chemotherapy and/or radiation.

If untreated, most cancers eventually cause death; cancer is one of the leading causes of death in developed countries. Most cancers can be treated and many cured, especially if treatment begins early. Many forms of cancer are associated with environmental factors, which may be avoidable. Cigarette smoking leads to more cancers than any other environmental factor.