The space where the bum ends and the upper thigh meets. A nice Nitch constitutes an overhang of the bum. A poor Nitch is where the bum leads directly into the thigh. A Nitch test can be done to determine the quality of ones Nitch. Simply put a pencil underneath your bum cheeks. If your butt holds the pencil you've got a sweet Nitch. If its doesn't well your ass sucks. Larger objects can be placed in the Nitch to ascertain the sweetness of a nice Nitch.
Girl: "I've got the nicest ass around"
Guy: "Prove it, do the Nitch test"