I agree most of us don't actually "learn in terms of these categories". We learn by seeing lots of positions and move-sequences with similar patterns, and internalizing those patterns.

But if you're writing a book sorted by theme, you need some chapter headlines! LeMoir's are perfectly good, and we don't need to memorize them anyway.

I don't recall hearing the expression "X-ray" at all when I was younger (i.e. when dinosaurs roamed the earth) but I became aware of it some years ago in contexts such as, say, White has a rook on e1 and Black's queen is on e7, but there is some piece clutter between the two; the white rook could be said to be exerting "X-ray pressure" on the black queen, which is a tactical weakness where it stands, an accident waiting to happen, just as a loose piece often is.

Hope this helps.

Judging from the examples, it seems that Silman considers the following to be an X-ray:

White piece A and a Black piece B are opposing each other along a rank (or file, or diagonal). White then moves piece C to a square behind B on the same rank. Even though C is not directly guarded by any White piece, it is indirectly guarded by A.

I don't know if this is correct, and I don't think I have ever learned a name for this theme (not even in my native language). But it seems to be Silman's interpretation of an X-ray.

This is quite different from a skewer, where White piece A threatens Black piece B along a rank (or file, or diagonal), and when B moves, A can capture piece C further behind on the same rank.

So, if I have understood Silman correctly, this is an X-ray:


1.Bf6+

while this is a skewer:


1.Bb2+