Mouth cancer is any cancer that develops in any of the parts of the mouth. This includes the bottom of the mouth, the top of the mouth, the tongue, the lips, the gums, or the inner lining of the cheeks. Mouth cancer may also be known as oral cancer or oral cavity cancer. It most commonly affects the lips and tongue. Mouth cancer is just one of a few different types of cancers that are considered head and neck cancers. It is estimated that about 10 out of 100,000 adults will develop mouth cancer.