Which condition involves the spread of cancer to other parts of the body?

Metastasis is the correct choice because it specifically refers to the process by which cancer cells spread from the original (primary) location to different parts of the body, often through the bloodstream or lymphatic system. This term is crucial in oncology, as the identification of metastasis indicates a more advanced stage of cancer and can significantly impact treatment options and prognosis.

In the context of the other terms, adenopathy refers to the enlargement of lymph nodes, often due to infection or malignancy, but does not specifically denote the spread of cancer. Staging is a method used to classify the extent and severity of cancer within the body, but it does not describe the actual movement of cancer cells. Infiltration may refer to the penetration of cancer into surrounding tissues but does not encapsulate the broader process of spreading to distant sites like metastasis does. Thus, the focus on spreading to other areas confirms that metastasis is the right answer.