The Back Bay is a Boston
neighborhood bordered by the Charles River
on the north, downtown and Beacon Hill
to the east, the South End
to the south, and the Fenway
to the west.
The name refers to the fact that the area really was a bay at one point (a putrid marsh, more accurately) -- the whole area was filled in during the late nineteenth century. As a relatively late addition to Boston, the Back Bay has the strange distinction of being laid out on a grid, in stark contrast to the winding streets of downtown Boston.
Commonwealth Avenue, which runs through the center of the Back Bay, was modeled after the Champs-Élysées in Paris and features a long park down the middle. It's part of Boston's Emerald Necklace.