The National Cancer Institute defines malignant mesothelioma as "as a rare form of cancer in which cancer cells are found in the sac lining the chest, the lining of the abdominal cavity or the lining around the heart." The medical names for these areas are the pleura, the peritoneum and the pericardium respectively and the lining around these organs is called the "mesothelium".

Malignant Peural mesothelioma therefore, is cancer which is found in the lining of the chest. Malignant Pleural mesothelioma. If Mesothelioma is found in the abdominal cavity it is known as peritoneum mesothelioma. Mesothelioma which is found in the lining of the heart is known as pericardium mesothelioma.

When mesothelioma cancer spreads, the cells of the mesothelium become abnormal. These malignant cells begin to divide and spread without order and can begin to affect organs and tissues in the immediate area. This often leads to complications in the surrounding organs.