Hypo melanistic or Hypo refers to Leopard Geckos which exhibit reduced melanin or black coloring. To be considered a Hypo rather than a High Yellow a leopard gecko must have 10 or fewer spots on its body (excluding the head and tail). If the gecko is all yellow with no black body spotting, it would be considered a Super Hypo.
The Hypo melanistic comes trough line breeding geckos that exhibit the least amount of spotting. Years of this method gives us a gecko with very little spotting.