Brain cancer: Cancer of the central information processing center of the body. Tumors in the brain can be malignant or benign, and can occur at any age. Only malignant tumors are cancerous. Primary brain tumors cancer initially forms in the brain tissue. Secondary brain tumors cancers are cancers that have spread to the brain tissue (metastasized) from elsewhere in the body. Secondary brain cancer is named for the organ or tissue in which the cancer begins, such as lung cancer with secondary brain metastasis.