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From 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]