An oncologist is a doctor who specializes in diagnosing and treating different forms of cancer. Hearing that a friend or family member has to visit an oncologist is never good news.
Oncology is the medical treatment of cancer, and an oncologist is someone who has trained to practice oncology. Cancer is a disease that involves the abnormal growth of normal cells into masses, or tumors, and the word oncologist includes the prefix onco-, which is used in medical words to mean "tumor." It stems from the Greek root onkos, or "mass."