Cancer is a type of neoplasia (meaning new growth) made up of abnormal cells that are often, but not always, consolidated into a mass (a swelling) called a tumor, or neoplasm. The common characteristic of all neoplastic cells is the absence of normal growth control mechanisms. Normal, healthy cells grow and reproduce only to replace cells that have died or, in young animals, to support ordinary growth and development. Neoplastic cells have no such restraints; they keep growing and reproducing even when there is no need for new cells, and many are able to invade and damage or destroy nearby healthy cells. Neoplasms may be benign or malignant. Benign tumors form a mass but do not spread to other areas of the body. Benign tumors can cause damage to tissues and need to be treated or removed if they grow to a size that compresses healthy tissue. Malignant neoplasms are more aggressive and have the ability to invade surrounding tissues or spread to distant sites (called metastasis). This behavior makes them more difficult to treat and remove. The term cancer is usually used for malignant neoplasms.
Any tissue in a body can develop neoplastic cells; no area of a body is immune. Neoplasms are named based on the type of cell or organ in which they develop. For example, hepatocellular carcinoma is a specific type of cancer involving the liver.