The state of a character being completely uncovered on the torso, or more literally front and back in a line from the character's neck to their crotches.
Note that this means that clothing/accessories worn on the extremities don't factor into whether a character is nude or not. Thus gloves, thighhighs, hats, detached sleeves, and the like do not matter in a character's state of nudity.
This also means items that aren't necessarily clothing can be used to make a character no longer count as nude. In images where towels, aprons, very long hair, bandages, and etc. are being used to cover the torso, don't tag the character as nude. Instead, use the format naked_*, where * is the item being used to cover the body. Naked apron and naked towel are examples.
- Items on a character's torso that are only for the purposes of accessorizing don't count as covering. This includes items such as necklaces, shoulder_pads, neckties, belts, garter_belts, collars, or other such items. Note that if these items are quite obviously purposefully being used as cover (post #1352273 for belts for example), use the same rule as any item that isn't clothing being used to cover the body and the naked_* tag format from above.
- A character "wearing" paint or chocolate or other such substances. Since these do not actually cover the body in a meaningful way, the character in question is still nude.
If a character such as a mermaid or other monster girl has inhuman body parts that are typically unclothed (e.g. below the waist), then count that region when determining total nudity. (i.e. Tag naked mermaids nude, not topless.)
The topless and bottomless tags are exclusive from the nude tag for a character in an image. Use topless when only the top part of the torso is uncovered, and bottomless when only the bottom part is. Nude is for when both parts are uncovered, so the tags can't be used for the same character in an image.
source: danbooru wiki