In medicine, malignant is a term referring to a condition that is dangerous to health.
A malignant tumor (cancerous tumor) is one that is invasive and can spread to other parts of the body. In contrast, tumors that stay localized and don't spread are called benign. Benign tumors may grow quite large and can do damage, but they do not usually spread through the bloodstream or lymph vessels to other parts of the body.
While the term malignant is often used interchangeably with cancer, not all malignant conditions are cancerous. For example, we use the word malignant hypertension to describe blood pressure that is dangerously high, but in this context it has nothing to do with cancer. Likewise, the condition malignant hyperthermia describes a dangerous situation in which a high fever develops during surgery with general anesthesia.