From Leopard Gecko Wiki
White Side is a gecko who expresses white (or very clear) coloring on his sides, the genetic of this trait is actually unproven, at this time he work as a line bred trait.
It is normal for a leopard gecko to have some white/faded color on their sides, these do not count as white sides despite what people say. It is generally thought, to count as a white side, the top of the white must be above the earhole, much like the first image below from EHgekkos and the fourth image from JBreptiles. These are great examples of White Sides.
Who is Working With Them?
White Side Images
White Side Hypo one time shedding(EH-Gekko's)
White Side Super Hypo hatchling(EH-Gekko's)
White Side Enigma (JBReptiles)
White Side]] JBR RAPTOGLOW (JBReptiles)