The Collaborative International Dictionary of English v.0.48:

Hornbeam \Horn"beam`\, n. [See Beam.] (Bot.)
   A tree of the genus Carpinus (Carpinus Americana), having
   a smooth gray bark and a ridged trunk, the wood being white
   and very hard. It is common along the banks of streams in the
   United States, and is also called ironwood. The English
   hornbeam is Carpinus Betulus. The American is called also
   blue beech and water beech.
   [1913 Webster]

   Hop hornbeam. (Bot.) See under Hop.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hornbeam
    n 1: any of several trees or shrubs of the genus Carpinus