Synonyms: Malignancy

Medical Specialties: Family practice, Internal medicine, Oncology

Clinical Definition

Cancer is not a single disease, but the general name that applies to more than 100 diseases. Each cancer has in common abnormal cells that grow in an uninhibited way and in the process, possibly forming tumors (i.e., abnormal growth of tissue). Cells become cancerous due to deoxyribonucleic acid (DNA) damage. In every cell, DNA is the director of activities; when it becomes damaged and is not repaired, the cell does not die as it is supposed to. The cell then begins making new cells, all with the same abnormal DNA. 

In Our Own Words

Some cancers are associated with high mortality, and overall, cancer is second only to heart disease as a leading cause of death in the United States. Cancer is not one entity, but includes more than 100 different diseases.


In cancer, abnormal cells begin to grow in an uncontrolled way due to DNA damage that cannot be repaired. Usually, cancer cells form a mass of tissue, or a tumor (notable exceptions include certain forms of leukemia or blood cancer). The tumor isn’t always obvious when small, however, which can make screening and detection a challenge.


The spread of cancer, or metastasis, starts when the cancer cells from the original site invade the blood or the lymph vessels. Cancers affecting different parts of the body grow at different speeds and need varying treatments. 

Relevant Conditions
Common Types
Side Effects
  • Vary, depending on cancer type
  • Fever
  • Fatigue
  • Weight loss
Share this article
  • Harvard Health Publications. "Harvard Dictionary of Medical Terms." Accessed August 2013.
  • American Cancer Society. "Signs and Symptoms of Cancer." Accessed August 2013.
  • American Cancer Society. "What Is Cancer?" Updated March 2013. Accessed August 2013.
Keep Reading

Investigate your bodys signs and signals.
Try Symptom Checker