Metastasis is the process by which cancer cells spread from the original (primary) tumor to other parts of the body, forming new (secondary) tumors. This occurs when cancer cells invade surrounding tissues, enter the bloodstream or lymphatic system, and travel to distant organs. Metastatic tumors are often more challenging to treat and can significantly affect a patient's prognosis. Understanding metastasis is crucial for developing effective cancer therapies and improving patient outcomes.