A pupil is an old-fashioned word for a young person attending school, or it can be that black dot in the center of your eye's iris. As a pupil, your pupils may dilate when your teacher dims the lights.
If you are a pupil, you are a learner enrolled in an educational institution. As a pupil, you have mastered the act of looking interested while thinking about the beach. Your pupil is also the aperture or opening in the center of your eye that looks like a big black dot. Dilating pupils are a very subtle sign that you are interested, lying, or pretending — or just trying to see in poor lighting.