The term "malignancy" refers to cancerous cells that usually have the ability to spread, invade, and destroy tissue. Malignant cells tend to have fast, uncontrolled growth 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.
ReferencesGoldman L, Ausiello D. Cecil Textbook of Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:1345-1348.