XYZ-Wing
An XYZ-Wing is a three-cell ALS-XZ construction. A trivalue cell {X,Y,Z} sees bivalue cells {X,Z} and {Y,Z}; all three together contain only X, Y, and Z. At least one of the three cells must contain Z, so an external Z candidate that sees every Z occurrence in the construction can be removed.
What is an XYZ-Wing?
The three cells form a bent almost restricted set: three cells with three distinct digits and the required visibility restrictions. Unlike XY-Wing, Z may be placed in the trivalue cell itself. That third case is part of the proof and is why the target must see all three Z occurrences.
How to spot it
Find a {X,Y,Z} cell and two bivalue cells {X,Z} and {Y,Z} that each see it. Check that any proposed Z target sees every cell in the construction that contains Z as a candidate. A similar-looking three-cell arrangement is insufficient when that visibility is missing.
The logic
If the trivalue cell is X, the {X,Z} cell is Z. If it is Y, the {Y,Z} cell is Z. If it is Z, Z is already in the trivalue cell. Thus Z occurs in at least one of the three cells in every case, and an external candidate seeing all three possible sources cannot be Z.