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]

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "boulevard":
   Autobahn, US highway, alley, alleyway, arterial, arterial highway,
   arterial street, artery, autoroute, autostrada, avenue,
   belt highway, blind alley, bypass, byway, camino real, carriageway,
   causeway, causey, chaussee, circumferential, close, corduroy road,
   county road, court, crescent, cul-de-sac, dead-end street, dike,
   dirt road, drag, drive, driveway, expressway, freeway, gravel road,
   highroad, highway, highways and byways, interstate highway, lane,
   local road, main drag, main road, mews, motorway, parkway, path,
   pave, paved road, pike, place, plank road, primary highway,
   private road, right-of-way, ring road, road, roadbed, roadway,
   route nationale, row, royal road, secondary road, speedway,
   state highway, street, superhighway, terrace, thoroughfare,
   through street, thruway, toll road, township road, track, turnpike,
   wynd