Malignant (from the Latin roots mal- = "bad" and -genus = "born") is a medical term used to describe a severe and progressively worsening disease. The term is most familiar as a description of cancer. A malignant tumor may be contrasted with a non-cancerous benign tumor in that a malignancy is not self-limited in its growth, is capable of invading into adjacent tissues, and may be capable of spreading to distant tissues (metastasizing), while a benign tumor has none of those properties. Malignant tumor is synonymous with cancer. Uses of "malignant" in oncology:

• Malignancy, malignant neoplasm, and malignant tumor are synonymous with cancer
• Malignant ascites
• Malignant transformation

Non-oncologic disorders referred to as "malignant":

• Malignant hypertension
• Malignant hyperthermia
• ''Malignant otitis externa
• Malignant tertian malaria (Malaria caused specifically by Plasmodium falciparum)
• Neuroleptic malignant syndrome