A Civil Surgeon is a medically trained and licensed physician who have no less than 4 years' experience and is designated by USCIS to perform immigration medical exams. The physician can only designated to be the civil surgeon in the state where he or she is licensed and practicing medicine.
USCIS requires all the green card applicants in the U.S. to provide a medical examination. The medical exam must be done by a civil surgeon. Other doctor in the U.S. that are not authorized by USCIS's medical exam will not be recognized. USCIS has provide a Find A Doctor website that listing all the civil surgeons by zip code and address.
Civil surgeons and Panel physician
Doctors that authorized to perform immigration medical examinations outside the U.S. and its territories are called panel physician. Applicants apply green card outside the U.S. will go to a panel physician to obtain a immigration medical exam report as they need to bring it to the interview at a U.S. embassy or consulate abroad. Different with civil surgeons that are designated by the USCIS, panel physicians are designated by the Department of State. You can find the authorized physicians here.