The Collaborative International Dictionary of English v.0.48:

Pediment \Ped"i*ment\, n. [L. pes, pedis, a foot. See Foot.]
   (Arch.)
   Originally, in classical architecture, the triangular space
   forming the gable of a simple roof; hence, a similar form
   used as a decoration over porticoes, doors, windows, etc.;
   also, a rounded or broken frontal having a similar position
   and use. See Temple.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pediment
    n 1: a triangular gable between a horizontal entablature and a
         sloping roof