The term 'PART OF AN EYEBALL' can be broken down into two crossword solutions: UVEA (4 letters) and IRIS (4 letters). The uvea, also known as the uveal tract or uveal layer, is the middle layer of the eye and consists of the choroid, ciliary body, and IRIS. The IRIS is the colored part of the eye that controls the amount of light entering the eye. It is located between the cornea and the lens and is responsible for controlling pupil size.