A tumor was an abnormal proliferation of cells in living tissue or organs.
In 2151, Doctor Phlox found interstitial tumors in Nadet's system. He recommended a series of cytolytic injections for her. (ENT: "Terra Nova")
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")