This is the really hot little space between a slender, well shaped young woman's upper thighs. It's the gap under her vagina between her legs. At the beginning of James Bond movies, where you see sexy chick's silhouettes, this is where guys are focused if not on the tits ass, or legs. If a girl isn't skinny, her thighs will probably touch when she stands or walks. This is not as desirable.
That girl at the gym was amazingly in shape. TDT (thighs don't touch) all day.