What is a metastasis?

A "metastasis" is the medical term for deposits or clumps of tumor cells that are found to have spread away from the primary site or origin of a tumor. An example would be a lung cancer that spreads or "metastasizes" to the liver, brain or lymph nodes. Tumors metastasize by spreading through the blood stream or through tiny drainage channels which lead to glands known as lymph nodes.

In addition to the previous answers, one of the terms that you may hear when healthcare providers are talking about cancer is 'mets.' For instance, "lung cancer with mets to the brain." This is short for metastasis and just means that, as the other answers have explained, that the cancer has spread to other organs or sites. The plural of metastasis is metastases so since both singular and plural are a bit of a tongue twister, you may hear mets.  

For those of you who like to know where words came from, metastasis is derived from the Greek word meaning displacement meaning "shift" or "movement."