An iris was the colored area surrounding the pupil in an eye.
All Betazoids had black-colored irises. (TNG: Various)
All Vorta had lavendar-colored irises. (DS9: Various)
In 2374, The Doctor fabricated an eye to replace Seven of Nine's Borg eyepiece. He showed particular pride in his ability to match the pigment of the iris in the fabricated eye to that of Seven's Human eye. (VOY: "The Gift")