The pupil of Species 8472
A pupil is the dark area of the eye surrounded by the iris. The function of the pupil is to control the amount of light allowed to pass into the eye.
The dilation of one's pupils could be an indicator of dishonesty. (TNG: "Up The Long Ladder") Talaxians and Humans both experienced pupil dilation when attracted romantically to others. (VOY: "Parturition", "Revulsion", "Unimatrix Zero") Some species such as the Mylean had naturally dilated pupils, which made them intolerant of bright light. (VOY: "Scientific Method")
Alternately, the contraction of one's pupils could be an indicator of illness. In 2370, Doctor Julian Bashir noted that Elim Garak's pupils were contracted when the Cardassian was experiencing headaches. (DS9: "The Wire")