When a character is nude in their natural state and anatomically_correct but not visibly aroused* or posed in a sexual** manner.
*(As in fully_sheathed, not spreading or dripping bodily_fluids. **such as focusing attention to their genitalia)
Typically applied to feral-bodied characters wearing little to no clothing, regardless of whether they are sapient or not.
Anthro characters, as they wear clothing more often then not, should be tagged with casual_nudity or similar.