The correct definition of "metastasis" is that it is the process by which cancer cells spread through the blood or lymphatic system to other sites or organs in the body.

Metastasis is a critical step in the progression of cancer. Cancer cells can break away from the primary tumor and enter the bloodstream or lymphatic vessels. They can then travel to distant locations in the body and establish secondary tumors, also known as metastases. This process allows cancer to spread from its original site to other organs or tissues, making the disease more difficult to treat.

The other options provided in the question do not accurately describe the process of metastasis:

- Cells dividing rapidly under the influence of drugs is not specifically related to metastasis. It may refer to the effect of certain drugs on cell growth, but it does not capture the specific process of cancer cells spreading to other sites.

- The attachment of chromosomes to the spindle before moving to the anaphase poles describes a step in cell division called mitosis, specifically the metaphase stage. While mitosis is a normal cellular process, it is not directly related to metastasis.

- The successful inhibition of cancer cells dividing further does not capture the concept of metastasis. While inhibiting cancer cell division is an important aspect of cancer treatment, metastasis refers to the spread of cancer cells to other sites rather than the inhibition of their division.

Therefore, the correct answer is:

Cancer cells spread through the blood or lymphatic system to other sites or organs.