A doctor whose job is to give you medicine so you feel no pain during an operation, and to keep you safe while you are under. They sit by your head, watch your breathing and your heart, decide how deep you should go, and bring you back when the surgeon is finished. In American English this doctor is an anesthesiologist. Though actually, the map of jobs is not the same everywhere: in Britain the doctor who does this work is usually called an anaesthetist, but in the United States anesthetist often means a nurse with special training, not the medical doctor. The American doctor is the anesthesiologist. The field they study is anesthesiology. You meet the word in hospitals and on consent forms, not in everyday chat.