What is the gap that shows between a woman's abdomen and her bikini bottoms when a skinny girl lies down? ?

Sometimes when my wife lays down on her towel by the pool, the "waistband" of her bikini bottoms does not lie down flat against her stomach. A small (roughly 1/2") gap becomes visible. I keep giving her grief about it...and want to know what the slang terms are for this "gap". Thank you.