Source : Webster's Revised Unabridged Dictionary (1913)

Expansion \Ex*pan"sion\, n. [L. expansio: cf. F. expansion.]
   1. The act of expanding or spreading out; the condition of
      being expanded; dilation; enlargement.

   2. That which is expanded; expanse; extend surface; as, the
      expansion of a sheet or of a lake; the expansion was
      formed of metal.

            The starred expansion of the skies.   --Beattie.

   3. Space through which anything is expanded; also, pure
      space.

            Lost in expansion, void and infinite. --Blackmore.

   4. (Com.) Enlargement or extension of business transactions;
      esp., increase of the circulation of bank notes.

   5. (Math.) The developed result of an indicated operation;
      as, the expansion of (a + b)^{2} is a^{2} + 2ab + b^{2}.

   6. (Steam Engine) The operation of steam in a cylinder after
      its communication with the boiler has been cut off, by
      which it continues to exert pressure upon the moving
      piston.

   7. (Nav. Arch.) The enlargement of the ship mathematically
      from a model or drawing to the full or building size, in
      the process of construction. --Ham. Nav. Encyc.

   Note: Expansion is also used adjectively, as in expansion
         joint, expansion gear, etc.

   {Expansion curve}, a curve the co["o]rdinates of which show
      the relation between the pressure and volume of expanding
      gas or vapor; esp. (Steam engine), that part of an
      indicator diagram which shows the declining pressure of
      the steam as it expands in the cylinder.

   {Expansion gear} (Steam Engine). a cut-off gear. See Illust.
      of {Link motion}.

   {Automatic expansion gear} or {cut-off}, one that is
      regulated by the governor, and varies the supply of steam
      to the engine with the demand for power.

   {Fixed expansion gear}, or {Fixed cut-off}, one that always
      operates at the same fixed point of the stroke.

   {Expansion joint}, or {Expansion coupling} (Mech. & Engin.),
      a yielding joint or coupling for so uniting parts of a
      machine or structure that expansion, as by heat, is
      prevented from causing injurious strains; as by heat, is
      prevented from causing injurious strains; as:
      (a) A side or set of rollers, at the end of bridge truss,
          to support it but allow end play.
      (b) A telescopic joint in a steam pipe, to permit one part
          of the pipe to slide within the other.
      (c) A clamp for holding a locomotive frame to the boiler
          while allowing lengthwise motion.

   {Expansion valve} (Steam Engine), a cut-off valve, to shut
      off steam from the cylinder before the end of each stroke.

Source : WordNet®

expansion
     n 1: the act of increasing (something) in size or volume or
          quantity or scope [syn: {enlargement}] [ant: {contraction}]
     2: a discussion that provides additional information [syn: {enlargement},
         {elaboration}]
     3: adding information or detail [syn: {expanding upon}]