1) A tumour that has spread from its original (primary) site of growth to another site, close to or distant from the primary site. Metastasis is characteristic of advanced malignancies, but in rare instances can be seen in neoplasms lacking malignant morphology. -- 2004

2) The spread of cancer from one part of the body to another. A tumour formed by cells that have spread is called a "metastatic tumor" or a "metastasis." The metastatic tumour contains cells that are like those in the original (primary) tumor. The plural form of metastasis is metastases.