Malignant describes something harmful that grows worse and spreads, most commonly used in medicine to describe dangerous cancers. When a doctor discovers a malignant tumor, it means the cancer cells can invade nearby tissue and spread to other parts of the body. This is the opposite of benign, which describes tumors that stay in one place and don't spread.

Outside of medicine, malignant describes something evil or destructive that keeps getting worse. A malignant rumor might be a lie that spreads through a school, causing more damage as more people believe it. Someone with malignant intentions wants to inflict serious, lasting harm, working actively to cause destruction.

When something is malignant, it's actively dangerous, likely to grow stronger, and capable of causing serious damage if left unchecked. Understanding whether something is malignant or benign helps doctors decide on treatment and helps people recognize which problems in life might need quick, serious attention before they spread and become much harder to fix.