A trapped piece is one that has no safe square to move to and is about to be captured, because every escape leads to it being taken.
A piece is trapped when its retreat squares are all covered and it can’t be defended enough to survive. The attacker then wins it simply by playing the move that takes it, with no good answer for the other side.
Edge of the board and greedy captures are where it happens most: a bishop that grabs a poisoned pawn on b2 or h2, a knight chased into a corner, or a queen that strays too deep and finds its lines of escape cut off.
Trapping a piece is one of the cleanest ways to win material. The defensive lesson is the flip side — before you send a piece on a raid, always make sure it has a way home.
A piece with no safe square to escape to that is about to be captured, because every move or defence still loses it.
Usually by venturing too far for a pawn or being chased to the edge of the board, where their retreat squares get cut off one by one.
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