三白眼是指眼睛的虹膜部份比较小，眼白的面积比较大，所以眼球的虹膜除了左右两侧有眼白之外，上方或下方也露出眼白。虹膜偏上者（即露出下眼白）称为下三白，反之称为上三白，不过上三白比较罕见。

Sanpaku gan(三白眼) or Sanpaku(三白) is a Japanese term meaning “three whites,” which originated from a Chinese term. It is generally referred to in English as "sanpaku eyes" and refers to eyes in which the white space above or below the iris is visible.