A tumor was an abnormal proliferation of cell in living tissue or organs.
Under certain conditions, people were known to alter their blood pressure, lower their body temperature, and manifest tumors as a result of psychological stimuli. (DS9: "Things Past")
Cancer could be found in the form of a tumorous growth. Ptera, a Vhnori, had a cancerous tumor on her brain stem in 2371 until it was removed by The Doctor on the USS Voyager. (VOY: "Emanations")