Define metastasis.

Metastasis is defined as the spread of cancer cells from the primary site, where the cancer originated, to distant locations in the body. This process is a critical characteristic of advanced cancer and indicates a more serious progression of the disease.

When cancerous cells invade surrounding tissues and eventually enter the bloodstream or lymphatic system, they can travel to and establish new tumors in other organs or tissues. This ability to spread is what makes metastatic cancer more challenging to treat than localized cancer. Understanding metastasis is crucial for determining prognosis and treatment strategies, as well as for recognizing how far the disease has progressed.

The other definitions, while related to cancer, do not specifically describe metastasis. Local growth of cancer cells refers to tumor expansion without implying distant spread. The transformation of normal cells into cancerous ones describes the initiation of cancer rather than its spread. The visibility of symptoms in cancer typically denotes an advanced disease state but is not directly tied to the definition of metastasis.