Metastasis, or metastatic disease, is the spread of a cancer from one organ or part to another non-adjacent organ or part. The new occurrences of disease thus generated are referred to as metastases (sometimes abbreviated mets). It was previously thought that only malignant tumor cells and infections have the capacity to metastasize; however, this is being reconsidered due to new research.
Cancer occurs after a single cell in a tissue is progressively genetically damaged to produce cells with uncontrolled proliferation. This uncontrolled proliferation, mitosis, produces a primary tumor. The cells which constitute the tumour eventually undergo metaplasia, followed by dysplasia then anaplasia, resulting in a malignant phenotype. This malignant phenotype allows for intravasation into the circulation, followed by extravasation to a second site for tumorigenesis.
Some cancer cells acquire the ability to penetrate the walls of lymphatic and/or blood vessels, after which they are able to circulate through the bloodstream (circulating tumor cells) to other sites and tissues in the body. This process is known (respectively) as lymphatic or hematogeneous spread.
After the tumor cells come to rest at another site, they re-penetrate the vessel or walls and continue to multiply, eventually forming another clinically detectable tumor. This new tumor is known as a metastatic (or secondary) tumor. Metastasis is one of three hallmarks of malignancy (contrast benign tumors). Most neoplasms can metastasize, although in varying degrees (e.g., basal cell carcinoma rarely metastasize).
When tumor cells metastasize, the new tumor is called a secondary or metastatic tumor, and its cells are similar to those in the original tumor. This means, for example, that, if breast cancer metastasizes to the lungs, the secondary tumor is made up of abnormal breast cells, not of abnormal lung cells. The tumor in the lung is then called metastatic breast cancer, not lung cancer.Source: Wikipedia