the contractile aperture in the center of the iris of the eye; resembles a large black dot
"The doctor examined her pupil to check if it reacted properly to light."
a young person attending school (up through senior high school)
"The new teacher greeted every pupil in the classroom with a warm smile."
A learner under the supervision of a teacher or professor.
"The doctor explained that the pupil is the small black circle in the center of your eye that lets light into it."
The hole in the middle of the iris of the eye, through which light passes to be focused on the retina.
In plain English: A pupil is the black center of your eye that gets bigger when it's dark and smaller when there is bright light.
"The new teacher asked each pupil to raise their hand before speaking."
Usage: Use pupil when referring to the dark opening in an eye that controls how much light enters, and use student for people who are learning from teachers. Avoid confusing this term with pupillary distance, which is a specific measurement used by optometrists rather than a general synonym for either meaning.
The word "pupil" entered English via French and originally meant an orphan or a child. Its meaning shifted over time to refer specifically to the part of the eye that dilates in dim light.