The term 'Losing Tic-Tac-Toe Line' refers to a sequence of five moves in the game of Tic-Tac-Toe that results in a loss for the player who makes the first move. The five moves are represented by the crossword solutions OXO (3 letters), OOX (3 letters), XOO (3 letters), XOX (3 letters), and XXO (3 letters). This sequence of moves is known as the 'Losing Tic-Tac-Toe Line' because it guarantees a loss for the player who makes the first move, regardless of the opponent's strategy.