Nickname for the western part of the White House which contains the Presidential Offices: the Oval Office, the Cabinet Room, the Roosevelt Room, the Press Briefing Room, and the offices of the President's executive staff. Originally designed by Thomas Jefferson, the West Wing and its offices were not built until 1902, under President Theodore Roosevelt. With six children in the mansion, quarters were getting cramped, so architect Charles McKim doubled the living space for the First Family and moved the offices out of the central mansion. The Oval Office and Cabinet Room were made permanent additions in 1909.

Sources: http://www.whitehouse.gov/news/west-wing.html
http://www.pbs.org/wnet/whitehouse/timeline/1902.html
http://www.whitehousehistory.org/