A cancerous tumor.
A malignant tumor—in contrast to a benign tumor—is one that spreads throughout the tissues or organ in which it originates and expands to other parts of the body as well (metastasizes). A malignant tumor is usually life-threatening, causing death if it remains untreated. If treated, the spread of a malignant tumor may be slowed or even arrested. Depending on the amount of tissue damage prior to treatment, tissue or organ function may be compromised.