X-ray attack
A long-range piece exerts pressure through an enemy piece on the same line, threatening what lies beyond.
An x-ray attack is a long-range piece’s influence projected through an enemy piece on the same line. The piece in the middle blocks the immediate threat, but if it ever moves, the back piece comes under fire. The attacker may not threaten anything concrete right now, but the geometry of the position means the defender must consider the x-rayed target on every move.
The term overlaps with pin and skewer but is distinct from both. A pin nails a piece in place because the back piece is more valuable. A skewer forces a more-valuable front piece to move. An x-ray is a more general motif: the front piece might be of any value, but the back piece is the real target of the attacker’s gaze, and tactical opportunities arise when the front piece becomes loose, gets captured, or has to move.
The x-ray is also a defensive resource. A rook on a8 x-raying through its own piece on a4 to an enemy piece on a1 is defending the piece on a4 in a sense: if Black ever captures on a4, White recaptures via the x-rayed line. This use of the x-ray, supporting one’s own piece through an opponent’s, is the basis of many endgame combinations.
X-ray geometry is one of the first things strong players check on every move. Two long-range pieces on the same line, even with something between them, are almost always meaningful.