Images or animations depicting at least one character who isn't wearing any clothing. Generally, decorations such as piercings and jewelry such as bracelets, rings, and necklaces do not count as "clothing". In addition, bondage does not count as clothing, to a point. If, for example, a character is completely naked except for cuffs around their wrists and ankles, and a ball gag stuffed into their mouth, then that still counts as nudity. An elaborate full-body rubber harness, on the other hand, counts as clothing even if the character's genitals (and/or breasts) are exposed.
Such images do not necessarily have to be erotic in nature. The nude tag still applies if a character's genitals and/or nipples are not visible, and even if the character is drawn in a cartoonish manner so that they do not appear to have genitalia (featureless_crotch).