Metastasis is the process by which cancer cells spread from the original (primary) site to other parts of the body. This occurs when malignant cells break away from the primary tumor, travel through the bloodstream or lymphatic system, and form new tumors (secondary sites) in organs such as the lungs, liver, bones, or brain. Metastatic cancer is often more difficult to treat and may require systemic therapies like chemotherapy, targeted therapy, or immunotherapy. The presence of metastasis is an important factor in cancer staging and prognosis.