Simple Definition:
Metastasized is when cancer cells spread from the original tumor to other parts of the body. This can happen through the blood or lymphatic system [a network of vessels that helps the body get rid of waste fluids and fight infections]. For example, if breast cancer cells spread to the lungs, it is called metastasized breast cancer.
Very Simple Definition:
Metastasized is when cancer spreads to different parts of the body. For example, if breast cancer spreads to the lungs, it is called metastasized breast cancer.