The Collaborative International Dictionary of English v.0.48:

Extravagance \Ex*trav"a*gance\, n. [Cf. F. extravagance. See
   Extravagant, and cf. Extravaganza.]
   1. A wandering beyond proper limits; an excursion or sally
      from the usual way, course, or limit.
      [1913 Webster]

   2. The state of being extravagant, wild, or prodigal beyond
      bounds of propriety or duty; want of moderation; excess;
      especially, undue expenditure of money; vaid and
      superfluous expense; prodigality; as, extravagance of
      anger, love, expression, imagination, demands.
      [1913 Webster]

            Some verses of my own, Maximin and Almanzor, cry
            vengeance on me for their extravagance. --Dryden.
      [1913 Webster]

            The income of three dukes was enough to supply her
            extravagance.                         --Arbuthnot.

   Syn: Wildness; irregularity; excess; prodigality; profusion;
        waste; lavishness; unreasonableness; recklessness.
        [1913 Webster]

WordNet (r) 3.0 (2006):

extravagance
    n 1: the quality of exceeding the appropriate limits of decorum
         or probability or truth; "we were surprised by the
         extravagance of his description" [syn: extravagance,
         extravagancy]
    2: the trait of spending extravagantly [syn: extravagance,
       prodigality, profligacy]
    3: excessive spending [syn: extravagance, prodigality,
       lavishness, highlife, high life]