The Collaborative International Dictionary of English v.0.48:

Frustum \Frus"tum\, n.; pl. L. Frusta, E. Frustums. [L.
   fruslum piece, bit.]
   [1913 Webster]
   1. (Geom.) The part of a solid next the base, formed by
      cutting off the, top; or the part of any solid, as of a
      cone, pyramid, etc., between two planes, which may be
      either parallel or inclined to each other.
      [1913 Webster]

   2. (Arch.) One of the drums of the shaft of a column.
      [1913 Webster]

WordNet (r) 3.0 (2006):

frustum
    n 1: a truncated cone or pyramid; the part that is left when a
         cone or pyramid is cut by a plane parallel to the base and
         the apical part is removed