X-ray
An x-ray is an attack or defense that operates through an enemy piece along a file, rank, or diagonal.
The slider “sees through” the unit in the middle. When that unit moves or is captured, the piece at the far end is suddenly attacked or defended.
The queen on d1 looks through the knight on d6 at the rook on d8. The knight is a screen, not a shield. If it leaves the file, the rook is hit; if it is captured, the queen recaptures on the same line.
Doubled rooks x-ray each other through an enemy piece in between. That is why lining heavy pieces on an open file stays useful even when something sits in the way.