Metastasis: 1. The process by which cancer spreads from the place at which it first arose as a primary tumor to distant locations in the body.
2. The cancer resulting from the spread of the primary tumor. For example, someone with melanoma may have a metastasis in their brain, while a person with colon cancer might, fortunately, have no metastases.
Metastasis depends on the cancer cells acquiring two separate abilities -- increased motility and invasiveness. Cells that metastasize are basically of the same kind as those in the original tumor. If a cancer arises in the lung and metastasizes to the liver, the cancer cells in the liver are lung cancer cells. However, the cells have acquired increased motility and the ability to invade another organ.
The ancient Greeks used the word metastasis to mean "removal from one place to another." The plural of "metastasis" is "metastases."
The term "metastatic" may be used along with the names of specific cancers to refer to the fact that the cancer has spread beyond its site of origin and has, therefore, become a metastatic cancer. For example, "metastatic melanoma" refers to melanoma that has spread beyond the skin to distant organs.