A doctor; physician — a general term for someone who treats illness. In everyday speech it is often politely said as 'o-isha-san'.
子どもが熱を出したのでお医者さんに連れて行った。
My child had a fever, so I took them to the doctor.
町の医者は優しく説明してくれた。
The local doctor explained things kindly.
医者の診断で手術が必要だと言われた。
The doctor said that surgery was necessary.