The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. That which is expanded; expanse; extend surface; as, the
      expansion of a sheet or of a lake; the expansion was
      formed of metal.
      [1913 Webster]

            The starred expansion of the skies.   --Beattie.
      [1913 Webster]

   3. Space through which anything is expanded; also, pure
      space.
      [1913 Webster]

            Lost in expansion, void and infinite. --Blackmore.
      [1913 Webster]

   4. (Economics & Commmerce) an increase in the production of
      goods and services over time, and in the volume of
      business transactions, generally associated with an
      increase in employment and an increase in the money
      supply. Opposite of contraction.

   Syn: economic expansion. [1913 Webster +PJC]

   5. (Math.) The developed result of an indicated operation;
      as, the expansion of (a + b)^2 is a^2 + 2ab + b^2.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
      [1913 Webster]

   Note: Expansion is also used adjectively, as in expansion
         joint, expansion gear, etc.
         [1913 Webster]

   8. an enlarged or extended version of something, such as a
      writing or discourse; as, the journal article is an
      expansion of the lecture she gave.
      [PJC]

   9. an expansion joint. See below. [Colloq. or jargon]
      [PJC]

   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 Automatic 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:
      (a) A slide 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.
      (d) a strip of compressible material placed at intervals
          between blocks of poured concrete, as in roads or
          sidewalks.

   Expansion valve (Steam Engine), a cut-off valve, to shut
      off steam from the cylinder before the end of each stroke.
      [1913 Webster]

WordNet (r) 3.0 (2006):

expansion
    n 1: the act of increasing (something) in size or volume or
         quantity or scope [syn: expansion, enlargement] [ant:
         contraction]
    2: a function expressed as a sum or product of terms; "the
       expansion of (a+b)^2 is a^2 + 2ab + b^2"
    3: a discussion that provides additional information [syn:
       expansion, enlargement, elaboration]
    4: adding information or detail [syn: expansion, expanding
       upon]

Moby Thesaurus II by Grady Ward, 1.0:

222 Moby Thesaurus words for "expansion":
   access, accession, accomplishment, accretion, accrual, accruement,
   accumulation, acreage, addition, advance, advancement,
   aggrandizement, amplification, amplitude, appreciation, area,
   ascent, attenuation, augmentation, ballooning, ballyhoo, big talk,
   bigness, bloating, blossoming, blowing up, body, boom, boost,
   bottoming out, breadth, broadcast, broadcasting, broadening,
   buildup, bulk, burgeoning, burlesque, business cycle,
   business fluctuations, bust, caliber, caricature, circumfusion,
   continuum, cooling off, coverage, crescendo, crisis, depression,
   depth, development, developmental change, diameter, diffraction,
   diffusion, dilatation, dilation, dilution, dimension, dimensions,
   dispensation, dispersal, dispersion, dissemination, dissipation,
   distance, distension, distention, distribution, divergence,
   downturn, economic cycle, economic expansion, economic growth,
   edema, elaboration, elevation, emptiness, empty space, enhancement,
   enlargement, evaporation, evolution, evolutionary change,
   evolvement, evolving, exaggerating, exaggeration, excess,
   exorbitance, expanding economy, expanse, expatiation, explication,
   extension, extent, extravagance, extreme, field, flood,
   flourishing, flowering, fragmentation, furtherance, gain,
   galactic space, gauge, girth, gradual change, grandiloquence,
   greatening, greatness, growth, gush, height, heightening,
   high growth rate, hike, huckstering, hyperbole, hyperbolism,
   increase, increment, infinite space, inflation, inordinacy,
   interstellar space, jump, largeness, leap, length, low,
   magnification, magnitude, market expansion, mass, maturation,
   measure, measurement, mounting, multiplication,
   natural development, natural growth, nonviolent change,
   nothingness, outer space, overemphasis, overestimation, overkill,
   overstatement, peak, peaking, peppering, prodigality,
   productiveness, profuseness, progress, progression, proliferation,
   propagation, proportion, proportions, prosperity, publication,
   puffery, puffing up, radiation, radius, raise, range, reach,
   recession, recovery, ripening, rise, scale, scattering,
   scatterment, scope, sensationalism, shotgun pattern, size,
   slowdown, slump, snowballing, sowing, space, spatial extension,
   spattering, sphere, splay, spread, spreading, sprinkling, stretch,
   stretching, strewing, superficial extension, superlative, surface,
   surge, swelling, tall talk, touting, tract, travesty, tumescence,
   unfolding, up, upping, upsurge, upswing, uptrend, upturn, void,
   volatilization, volume, waxing, widening, width, working-out