The Collaborative International Dictionary of English v.0.48:

Pyramid \Pyr"a*mid\, n. [L. pyramis, -idis, fr. Gr. ?, ?, of
   Egyptian origin: cf. F. pyramide.]
   [1913 Webster]
   1. A solid body standing on a triangular, square, or
      polygonal base, and terminating in a point at the top;
      especially, a structure or edifice of this shape.
      [1913 Webster]

   2. (Geom.) A solid figure contained by a plane rectilineal
      figure as base and several triangles which have a common
      vertex and whose bases are sides of the base.
      [1913 Webster]

   3. pl. (Billiards) The game of pool in which the balls are
      placed in the form of a triangle at spot. [Eng.]
      [1913 Webster]

   4. (Finance) a fraudulent investment scheme in which the
      manager promises high profits, but instead of investing
      the money in a genuine profit-making activity, uses the
      money from later investors to pay the profits to earlier
      investors; -- also called pyramid scheme or pyramid
      operation. This process inevitably collapses when
      insufficient new investors are available, leaving the
      later investors with total or near-total losses of their
      investments. The managers usually blame government
      regulations or interference for the collapse of the
      scheme, rather than admit fraud.
      [PJC]

   Altitude of a pyramid (Geom.), the perpendicular distance
      from the vertex to the plane of the base.

   Axis of a pyramid (Geom.), a straight line drawn from the
      vertex to the center of the base.

   Earth pyramid. (Geol.) See Earth pillars, under Earth.


   Right pyramid (Geom.) a pyramid whose axis is perpendicular
      to the base.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pyramid scheme
    n 1: a fraudulent scheme in which people are recruited to make
         payments to the person who recruited them while expecting
         to receive payments from the persons they recruit; when the
         number of new recruits fails to sustain the hierarchical
         payment structure the scheme collapses with most of the
         participants losing the money they put in