A polite, somewhat formal term for a doctor. Rare in modern usage; more common terms are 医者 or 先生.
医伯の診断によれば、特に問題はないとのことです。
According to the doctor's diagnosis, there is no particular problem.
Composed of 医 (medicine) and 伯 (count, earl; here used as a respectful suffix). The exact historical derivation is uncertain, but it likely arose as a polite term for a physician.