A moderately-sized rectangular gap between clothing that exposes the character's stomach. The total amount of exposed skin should equal no more than half the stomach’s surface; with waistline bottomwear the gap ends slightly above the navel, though it can shift to accommodate high-waist clothing.
Use midriff when there are regular clothes above (e.g. a crop top) and below (e.g. pants) the stomach, but the stomach is bare or visible beneath see-through clothes clothing. "Regular" is not limited to real apparel.
Note: Midriff may still apply to otherwise disqualifying clothing if it is underwear worn as outerwear, such as sports bras, camisoles, tank tops worn with panties, etc.