The Collaborative International Dictionary of English v.0.48:

Backboard \Back"board`\, n. [2d back, n. + board.]
   [1913 Webster]
   1. A board which supports the back when one is sitting;
      specifically, the board athwart the after part of a boat.
      [1913 Webster]

   2. A board serving as the back part of anything, as of a
      wagon.
      [1913 Webster]

   3. A thin stuff used for the backs of framed pictures,
      mirrors, etc.
      [1913 Webster]

   4. A board attached to the rim of a water wheel to prevent
      the water from running off the floats or paddles into the
      interior of the wheel. --W. Nicholson.
      [1913 Webster]

   5. A board worn across the back to give erectness to the
      figure. --Thackeray.
      [1913 Webster]

WordNet (r) 3.0 (2006):

backboard
    n 1: a raised vertical board with basket attached; used to play
         basketball; "he banked the shot off the backboard" [syn:
         backboard, basketball backboard]
    2: a board used to support the back of someone or something