The Collaborative International Dictionary of English v.0.48:

hardboard \hard"board`\ n.
   a cheap hard material made from wood chips that are pressed
   together and bound with synthetic resin to form sheets, used
   in construction and various other purposes; -- called also
   particle board and chipboard.

   Syn: chipboard.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

hardboard
    n 1: a cheap hard material made from wood chips that are pressed
         together and bound with synthetic resin [syn: chipboard,
         hardboard]