The character's stomach is visible, and there is no visible navel.
The tag applies both when the character should have a navel (e.g. the character is a human, a mammal animal type, etc.), or when the character shouldn't have a navel (the character is an android, alien, non mammal type, magical creature, etc.). In the former case, tag artistic error as well.
The following tags are aliased to this tag: no_belly_button and no_bellybutton (learn more).