Tumors are defined as abnormal swelling or enlargement caused by excessive growth and cell division; usually cells grow and divide to produce new cells in a controlled and orderly manner. Although often used as a synonym for neoplasms, the word tumor is not synonymous with cancer. A tumor is a mass of tissue formed by the accumulation of abnormal cells. Usually, the cells in your body age, die, and are replaced by new cells. Tumor cells grow, although the body does not need them, and unlike normal old cells, they do not die. As this process progresses, the tumor continues to grow as more cells are added to the mass.