The Collaborative International Dictionary of English v.0.48:

Boulevard \Bou"le*vard`\, n. [F. boulevard, boulevart, fr. G.
   bollwerk. See Bulwark.]
   [1913 Webster]
   1. Originally, a bulwark or rampart of fortification or
      fortified town.
      [1913 Webster]

   2. A public walk or street occupying the site of demolished
      fortifications. Hence: A broad avenue in or around a city.
      [1913 Webster]

WordNet (r) 3.0 (2006):

boulevard
    n 1: a wide street or thoroughfare [syn: avenue, boulevard]