The clear see-thru opening in a gal's crotch when she stands with her legs together. The size of this crotch cavity is usually described in terms of the number of fingers you could slip in to fill the gap. (ie. two-finger shizok). Shizok's are more readily visible when gal's wear tight jeans.
"That broad must like riding the pony cuz she has about a three finger shizok!"