The open space created between a thin or fit woman's upper thighs when she stands straight up with her feet close together. Considered a reliable indicator of a low body fat to weight ratio for women. More voluptuous women do not have a crotch gap due to the flesh of their thighs closing the gap. Often associated with the term "cameltoe".
When the backlight revealed her crotch gap through her dress, I knew she was the one for me.