The term "100% eyeclean" (or sometimes just "eyeclean") describes how clear a diamond appears to the naked eye.
It is not a characteristic featured with a grading report, such as a GIA or IGI certificate.
While the overall clarity of a diamond is graded on a clarity scale between FL (Flawless, extremely rare and free of any marks or inclusions) down to I3 (3 X Included, with multiple, easily visible inclusions and blemishes), this clarity scale does not in isolation differentiate between the nature of inclusions between stones of the same clarity.
Expressed another way, when placing two round cut diamonds of one carat weight each, side by side, both of which have been graded by the GIA as clarity scale VS1 ("Very Slightly Included") it is often possible to identify marks within one stone (diamond A), particularly near the table (the top area on display) which are less evident in the other (diamond B).
If the inclusions in diamond B are more scattered, lighter in tone and more concealed within the stone, such that they cannot be seen without magnification, this diamond maybe considered within the trade as being "100% eyeclean".
Note that eyeclean is not related to the diamond's colour.
What you will see under 10X magnification
What you will see with the naked eye, up close
What you will see with the naked eye, up close: 100% eyeclean
Diamond A has a visible inclusion near the centre of the table surface. Diamond B (eyeclean) is free from visual inclusions.
It is generally rare to find a 100% eyeclean diamond below clarity scale Si1.
However, a diamond with clarity of VS1 which is not eyeclean may be less desirable than an eyeclean diamond of clarity Si1.
Nevertheless the Si1 clarity diamond should be more affordable in this instance than the VS1 stone, although one also needs to factor in various other characteristics of each specific diamond in ascertaining it's value.