Metastasis (Greek: displacement, μετά=next + στάσις=placement, plural: metastases), sometimes abbreviated mets, is the spread of a disease from one organ or part to another non-adjacent organ or part. Only malignant tumor cells and infections have the capacity to metastasize.

Cancer cells can "break away", "leak", or "spill" from a primary tumor, enter lymphatic and blood vessels, circulate through the bloodstream, and settle down to grow within normal tissues elsewhere in the body. Metastasis is one of three hallmarks of malignancy (contrast benign tumors). Kumar, Abbas, Fausto; Robbins and Cotran: Pathologic Basis of Disease; Elsevier, 7th ed. Most tumors and other neoplasms can metastasize, although in varying degrees, barring a few exceptions (e.g., Glioma and Basal cell carcinoma never metastasize).

When tumor cells metastasize, the new tumor is called a secondary or metastatic tumor, and its cells are like those in the original tumor. This means, for example, that, if breast cancer spreads (metastasizes) to the lung, the secondary tumor is made up of abnormal breast cells, not of abnormal lung cells. The tumor in the lung is then called metastatic breast cancer, not lung cancer.