The Collaborative International Dictionary of English v.0.48:

Coloboma \Col`o*bo"ma\, n. [NL. fr. Gr. ?, the part taken away
   in mutilation, fr. ? to mutilate.] (Anat. & Med.)
   A defect or malformation; esp., a fissure of the iris
   supposed to be a persistent embryonic cleft.
   [Webster 1913 Suppl.]