That is your anatomical waist. Technically, it's measured as the smallest circumference between the bottom of your xyphoid process (the thing where your ribs meet in the front of your chest) and the umbilicus (belly button!). Unless you have a weirdly shaped body due to unusual fat or muscle, this should be true for 99.9% of the population. The area you thought was your waist was actually your hips, which is defined as the largest protrusion area of the gluteal folds (the buns).