Currently
x-ray alias to xray and
crosssection alias cross_section
so none of them can be aliased anymore.

But shouldn't they all alias to x-ray?

Also,
http://danbooru.me/tag_alias?query=xray&commit=Search+Aliases
it says they're aliased, but if I tag something with x-ray it doesn't change to xray. ???

Probably. There might be some difference that I don't know about, but I think so.

I think cross section is the more precise term for this.
"x-ray" also implies, to me, something like unintentionally see through clothes (notwithstanding the correct definition, which would show you skeleton.)

7HS's post makes sense, although not quite a sufficient description. His post gave me this idea:

cross_section should be used to describe instances in images where you, the viewer of the picture, can see inside the girl, for the sole reason that the artist drew it that way.

xray should be used to describe pictures in which the girl or her clothes can actually be seen through within the world, like a male character looking at her with xray vision, or where her clothes suddenly become transparent, or something. In other words, it isn't just drawn that way for the viewer to see, but it actually IS that way within the situation.

And I guess xray would also be used for images containing actual xrays, in the medical context. I don't foresee such images being posted often.

This is the colloquial x-ray, you know, the Superman x-ray.

But really, you can't take this too literally. An actual cross section would be those images where actually theres a separate square with the inside view like post #252760 but even so, if it was a cross section, the penis would also be cross sectioned and that never happens. x-ray are those images where part of the insides can be seen through the body like post #33587 and cross section is already a subset of that. In practice they're being used as synonyms already so for simplicity's sake they should be synonyms.

I guess the thing closest to what we're looking for, yet still incorrect, would be a CAT scan but I think x-ray gives people the message more clearly.

internal_cumshot also seems to be the same thing as x-ray 90% of the times as x-ray is usually only used in the final internal cumshot.