The state of being unclothed. On Danbooru, this means the character's full torso, chest and back from neck to crotch, is uncovered by clothing.
Accessories don't count. If a character is wearing items that don't cover their chest or crotch - such as gloves, detached sleeves, thighhighs, hats, shoulder pads, necklaces, neckties, collars, belts, garter belts, and so on - they're still considered 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 as nude, not topless.)
Do not use nude together with tags describing partial nudity. Topless, bottomless, no bra, no panties, bare arms, bare back, bare shoulders, and midriff should not be used with the nude tag. The barefoot tag is still appropriate to use.