Tumor: An abnormal mass of tissue. Tumors can be benign or malignant (cancerous). There are hundreds of different types of tumors. Their names usually reflect the kind of tissue they arise in, and may also tell you something about their shape or how they grow. Diagnosis depends on the type and location of the tumor. Tumor marker tests and imaging may be used; some tumors can be seen (for example, tumors on the exterior of the skin) or felt (palpated with the hands). Treatment is also specific to the location and type of the tumor.