A doctor or physician — a professional who provides medical care, diagnoses and treats illnesses (formal term).
外科医師は複雑な手術を成功させた。
The surgeon successfully performed a complicated operation.
医師の指示に従って薬を飲んでください。
Please take the medicine according to the doctor's instructions.
町の医師は予約が取りにくいほど忙しい。
The town's doctor is so busy that it's hard to get an appointment.