A “fossa”, in anatomy, is a gap, a hollow area or a depressed area.
A good example of a fossa is the axillary fossa, which is the indent in the body under the arm, running down the side of the body from the point where the arm joins the shoulder.
The man lying on his back on the bench is showing his axiliary fossa. It’s the hollow running down his side, starting from his armpit.
So those are the three elements to the thigh gap, in anatomese:
Together they make up this.