The little space between a girls upper thighs and just below her cooter as she stands or walks with legs together. The open space forms the shape of an upside down triangle. It is usually reserved to fit/thin girls as chubby thighs tend to crush the happy gap. Fall into the gap.
Holy shit did you see that happy gap! It was so gapalicious that I could light it on fire and jump through it on my mini-bike.