1A doctor or physician — a professional who provides medical care, diagnoses and treats illnesses (formal term; colloquially, 医者 is used more often).
その病院には経験豊富な医師が何人もいる。
That hospital has several experienced doctors.
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.