Synonyms: Cancerous, Cancer

Medical Specialties: Oncology

Clinical Definition

Abnormally growing cells can become cancerous, or malignant. Malignant cells may travel from their point of origin and can be fatal if left untreated. 

In Our Own Words

Cancer generally describes a group of abnormal cells that tend to divide more rapidly than most healthy cells or lack the natural life cycle that keeps things in balance. Once these cells start dividing and growing out of control, they may form a tumor. The tumor may be benign, meaning non-cancerous, or the tumor can be malignant, meaning cancerous. Benign tumors cannot invade other tissues, while malignant tumors can.


Malignancy is extremely common; about one in every three Americans will develop some type of malignancy in his or her lifetime. There are more than 100 types of cancer. Malignant cells can be caused by many different factors, including genetics, lifestyle choices (e.g., smoking and diet) and exposures. Although all malignant cells are classified as cancer, they behave differently. Malignant cells are usually diagnosed through some sort of definitive test, such as a blood or urine test, an imaging test or a biopsy. 

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  • Cleveland Clinic. “Cancer Overview.” Accessed July 2013.
  • American Cancer Society. “What is Cancer?” Updated March 2012. Accessed July 2013.
  • Harvard Health Publications. “The 10 commandments of cancer prevention.” Accessed July 2013.
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