The term "malignancy" refers to the presence of cancerous cells that have the ability to spread to other sites in the body (metastasize) or to invade nearby (locally) and destroy tissues. Malignant cells tend to have fast, uncontrolled growth and DO NOT die normally due to changes in their genetic makeup.
Malignant cells that are resistant to treatment may return after all detectable traces of them have been removed or destroyed.
Black AR, Cowan KH. Cancer biology and genetics. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 25th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 181.
National Cancer Institute. NCI dictionary of cancer terms. Cancer.gov Web site.