The tissue that lines your lungs, stomach, heart, and other organs is called mesothelium. Mesothelioma is a tumor of that tissue. It usually starts in the lungs, but can also start in the abdomen or other organs. It can be benign (not cancer) or malignant (cancer.)
Malignant mesothelioma is a rare but serious type of cancer. Most people who get it have worked on jobs where they inhaled asbestos particles. After being exposed to asbestos, it usually takes a long time for the disease to form.
Sometimes it is hard to tell the difference between malignant mesothelioma and lung cancer. Your doctor uses imaging tests and a biopsy to make the diagnosis. Malignant mesothelioma is often found when it is advanced. This makes it harder to treat. Treatment may include surgery, radiation, and/or chemotherapy.
NIH: National Cancer Institute