Metastasis is a secondary malignant tumor (one that has spread from a primary cancer to affect other parts of the body); for example, a metastasis in the lungs may arise as a result of the spread of a cancer that originated in the skin. The term metastasis also applies to the process by which such spread occurs. The degree of malignancy of a tumor depends largely on its ability to invade surrounding normal tissue and on its ability to send metastases to other parts of the body.

Metastases can spread from one part of the body to another through the lymphatic system, in the bloodstream, or across a body cavity, such as that between the inner and outer layer of the peritoneal membrane in the abdomen.