An "eye-clean" diamond is one that has no imperfections visible to the unaided eye. They offer excellent value, being much less expensive than flawless (FL) or internally flawless (IF) diamonds, which are extremely rare and therefore command a higher price.
Imperfections in diamonds graded Slightly Included (SI) are often not visible to the unaided eye, making them excellent value for money, however the location of the inclusion is important. Therefore, it always advisable to speak to a diamond and jewellery consultant to check that the stone is eye-clean, if you're considering a diamond of this clarity grade.