Seeing inside or through a normally opaque barrier, as if by x-ray vision. Examples: skin, bone, armor, metal containers, concrete walls...
Typically rendered as a partially transparent overlay, or a cut-out. The internal and external views should always be overlapping. Rule of thumb: It's x-ray if Superman could see it with x-ray vision.
Use x-ray film for the images taken with actual x-ray machines (or any medical device that takes cross-section images).
Disambiguation
Wet, sheer, or clear clothing only qualifies as see-through clothes because it has natural transparency.
A cut-out internal view separated from the main image only qualifies as cross-section because it doesn't show the external barrier.
