That half of the board made up of the four files originally occupied by the Queen, Q-Bishop, Q-Knight and Q-Rook. The queenside stands to White°s left and Black°s right.
The a-d files. The left half of the board where the queens and the bishops, knights, and rooks next to them initially start.
The half of the board from which the queen starts. The a, b, c and d files.