The Collaborative International Dictionary of English v.0.48:

backwash \back"wash`\, v. t.
   1. To clean the oil from (wool) after combing. --Ash.
      [1913 Webster]

   2. (Chem.) To clean (a filter, a chromatography column, a
      water softener, etc.) by passing liquid through in the
      reverse of the normal direction of flow. This procedure
      dislodges particles which clog the pores of the filter or
      column packing, removing them from the filter or column.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

backwash \back"wash`\, n.
   1. The flow of water propelled backward by the propeller,
      paddle wheel, or oars of a boat.
      [PJC]

   2. (Aeron.) The backward flowing air within the wash of an
      airplane, caused mostly by the engine.
      [PJC]

WordNet (r) 3.0 (2006):

backwash
    n 1: the flow of air that is driven backwards by an aircraft
         propeller [syn: slipstream, airstream, race,
         backwash, wash]
    2: the consequences of an event (especially a catastrophic
       event); "the aftermath of war"; "in the wake of the accident
       no one knew how many had been injured" [syn: aftermath,
       wake, backwash]
    3: the wave that spreads behind a boat as it moves forward; "the
       motorboat's wake capsized the canoe" [syn: wake,
       backwash]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "backwash":
   Charybdis, Maelstrom, back stream, backcountry, backflow, backlash,
   backwater, backwoods, boondocks, bush, clout, condensation trail,
   contrail, countercurrent, counterflow, counterflux, eddy, exhaust,
   force, gulf, gurge, hinterland, impact, impress, impression,
   imprint, maelstrom, mark, print, reaction, recoil, reflex,
   refluence, reflux, regurgitation, repercussion, response, sticks,
   stream, swirl, track, twirl, up-country, vapor trail, vortex, wake,
   wash, whirl, whirlpool