Characters with a translucent skin or membrane covering their abdominal area instead of an opaque one, making their internal organs visible from the outside.
Alternatively this could also refer to robots or cyborgs with an integrated window or viewing port on their abdomen, making their endoskeleton and internal machinery visible from the outside.

Not to be confused with

• internal - The insides of a character is visible for the viewer's convenience rather than literally being visible in-universe.
• translucent_body - The character's entire body is translucent and not just their belly alone.
• exposed_organs - The organs are outside of the body or there's a gaping hole that is exposing the organs.