The Collaborative International Dictionary of English v.0.48:

Fastigiate \Fas*tig"i*ate\, Fastigiated \Fas*tig"i*a`ted\, a.
   [L. fastigium gable end, top, height, summit.]
   1. Narrowing towards the top.
      [1913 Webster]

   2. (Bot.) Clustered, parallel, and upright, as the branches
      of the Lombardy poplar; pointed.
      [1913 Webster]

   3. (Zool.) United into a conical bundle, or into a bundle
      with an enlarged head, like a sheaf of wheat.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fastigiate
    adj 1: having clusters of erect branches (often appearing to
           form a single column)