A student, especially a child at school; it also means the small dark circular opening in the center of the eye that lets in light.
The 'student' meaning comes from Latin *pupillus* ('ward, minor, orphan under protection'). The 'eye' meaning comes from Latin *pupilla* ('little doll'), because people saw tiny reflections of themselves in others' eyes.
Your pupil (student) and the pupil in your eye are actually two different Latin words that just ended up looking the same in English. The 'little doll' in your eye is literally the tiny reflection of someone standing in front of you.
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