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The game state at a particular moment, including pieces, side to move, and relevant special rights.

A legal action taken by one player on their turn to change the position on the board.

The player, White or Black, who has the right and obligation to make the next move.

The square occupied by a piece immediately before it begins a move.

The square occupied by the moving piece when a move has been made.

A move that follows every applicable rule and does not leave the player's own king in check.

A move that breaks a rule, especially one that leaves or places the player's own king in check.

A board state that could not have been reached through any sequence of legal moves.

A candidate move that follows basic piece movement but may still be illegal because it leaves the moving side's king in check.

A piece attacks a square or opposing piece when it could capture there according to its movement pattern.

A square controlled by an opposing piece because that piece could capture there under its movement rules.

A move that removes an opposing piece by occupying its square, except for the special en passant capture.

A capture answering an opponent's capture, usually by immediately taking the piece that just captured.

In technical analysis, a non-pawn move to an empty square that does not reset the fifty-move count.

A move that permanently changes a piece, right, or historically significant part of the position state.