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In medicine, malignant is a clinical term that means to be severe and become progressively worse, as in malignant hypertension. In the context of cancer, a malignant tumor can invade and destroy nearby tissue and may also spread (metastasize) to other parts of the body.

The word malignant comes the Latin combination of "mal" meaning "bad" and "nascor" meaning "to be born"; malignant literally means "born to be bad." Strictly speaking, a malignant tumor is one that possesses the ability to invade adjacent tissues and spread distantly. Just because a tumor is diagnosed as malignant by a pathologist does not mean that the patient will progress rapidly to death. More and more, patients with malignant tumors are living for many years after diagnosis and treatment. The actual prognosis depends of the type of tumor, its differentiation, and the stage (extent) of disease. The term malignant is typically applied to neoplasms that show aggressive behavior characterised by local invasion or distant metastasis.

The change of cells from benign to malignant behavior is called malignant transformation. This diagnosis is made by pathologic examination of tissues. Often the clincial signs and symptoms are suggestive of a malignant tumor. The physician, during the medical history examination, can find that there have been changes in size or patient sensation and, upon direct examination, that there has been a change in the lesion itself.

Malignant transformation is a synonym for progression to malignancy. Transformation may take place as a primary or a secondary process.

There are many causes of malignant transformation. The underlying commonality is genetic mutation either by inheritance or more commonly by acquiring mutations in one's DNA over time. Although malignant transformation may occur because of changes within the cell, it can be induced by inorganic toxic substances such as cadmium or arsenite and organics such as tobacco-specific nitrosamines. It is also thought that some malignant transformations are due to viruses such as the Epstein-Barr virus, although this is currently restricted to just a few cancer types. A more common cancer associated with viral infection is cervical cancer, which has been linked to the human papilloma virus.

Malignant qualities in cancer may be due to still multiplying cells, or cells that break off from the main tumour and enter the blood stream, being transported elsewhere where the cells that broke off are able to reproduce further, thus spreading the cancer.

Risk assessments can be done and are known for certain populations. One of the better known of these progression and risks is the progression of a nevus to melanoma.

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