In medicine, malignant is a clinical word that is used to describe an illness that kills a patient fast. A malignant tumor is a tumor that is able to invade nearby tissues and spread to tissues that are not nearby. Not all patients diagnosed with malignant tumors die, because of the improved treatments available. The actual prognosis depends on the type of tumor, its differentiation, and how bad the disease is. The term malignant is typically applied to neoplasms that show aggressive behavior characterized by local invasion or distant metastasis.