Historically, it was an addition to the city west of Van Ness Avenue (hence, "Western Addition"). The area was first developed around the turn of the 20th century as a middle-class suburb served by cable cars. Except for Hayes Valley, it survived the 1906 San Francisco earthquake with its Victorian-style buildings largely intact.
Today, the term Western Addition is generally used in two ways: to denote the development's original geographic area, or to denote the eastern portion of the neighborhood (also called the Fillmore District) that was redeveloped in the 1950s.
Those who use the term in the former sense generally consider its (relatively ill-defined) boundaries to be Van Ness Avenue on the east, Masonic on the west, California Street on the north, and Fell or Oak Street on the south. From there, it is usually divided into smaller neighborhoods such as Lower Pacific Heights, Cathedral Hill, Japantown, The Fillmore, Hayes Valley, Alamo Square, Anza Vista, and North Panhandle.
The San Francisco Association of Realtors defines the term more closely to the latter sense, treating it as "District 6D" (a goodly portion of this neighborhood is in board of supervisors district 5), bounded by Geary Boulevard in the north, McAllister, Fulton, and Golden Gate Streets on the south, Van Ness Avenue in the east, and Divisadero Avenue in the West. By this definition, the Western Addition is roughly synonymous with The Fillmore and Cathedral Hill.
After the Second World War, the Western Addition — particularly the Fillmore District — became a population base and a cultural center for San Francisco's African American community. Since then, urban renewal schemes and San Francisco's changing demographics have led to major changes in the economic and ethnic makeup of the neighborhood, as the Fillmore District suffered from crime and poverty while many other districts underwent significant gentrification. Today, many areas of the neighborhood are again solidly middle-class.