Metastasis is the process of development of secondary tumors (metastases) resulting from the spread of cells from the primary focus to other tissues.
Metastasis is the main criterion for tumor malignancy. It is the presence of metastases that makes a complete cure for a cancerous tumor impossible without the removal of metastatic nodules. Often tumors are incurable because of the presence of metastases in the vital organs (liver, brain, etc.).