The midriff is the region of the body (not just in front) between the chest and the waist. Use this tag when there are clothes above (e.g. a tank top) and below (e.g. pants, shorts, skirt), but the midriff is uncovered or visible beneath see-through clothing. Don't use this tag if the character is topless, naked, or wearing only underwear or a bikini. Often goes with navel.
source: danbooru wiki