Deflection is a tactical theme that forces an opponent's piece to leave a square, file, or diagonal it is currently occupying. This is usually done to remove a defender from a key square or piece.
Think of it as a distraction. You make a threat that the opponent must respond to, and by doing so, they are forced to stop protecting something even more important.
Deflection is a common way to deliver checkmate or win material. For example, if a King is the only piece defending a Queen, you might check the King to force it to move away. Once the King moves, the Queen is left unprotected and can be captured easily. Mastering deflection requires looking at what your opponent's pieces are currently defending. If you can lure that defender away, you can break through their defenses.
Deflection is often categorized as a "luring" tactic because it forces a piece to abandon its current duties. In many defensive setups, a single piece may be overworked, meaning it is responsible for guarding multiple squares or units at once. By applying pressure to that specific defender or offering an enticing sacrifice, you force the opponent to make a difficult choice: maintain the defense and face immediate loss, or accept the deflection and leave a critical area exposed.