The Collaborative International Dictionary of English v.0.48:

Opportunity \Op`por*tu"ni*ty\, n.; pl. Opportunities. [F.
   opportunit['e], L. opportunitas. See Opportune.]
   1. Fit or convenient time or situation; a time or place
      permitting or favorable for the execution of a purpose; a
      suitable combination of conditions; suitable occasion;
      chance.
      [1913 Webster +PJC]

            A wise man will make more opportunities than he
            finds.                                --Bacon.
      [1913 Webster]

   2. Convenience of situation; fitness. [Obs.]
      [1913 Webster]

            Hull, a town of great strength and opportunity, both
            to sea and land affairs.              --Milton.
      [1913 Webster]

   3. Importunity; earnestness. [Obs.] --Jer. Taylor.
      [1913 Webster]

   Syn: Occasion; convenience; occurrence.

   Usage: Opportunity, Occasion. An occasion is that which
          falls in our way, or presents itself in the course of
          events; an opportunity is a convenience or fitness of
          time, place, etc., for the doing of a thing. Hence,
          occasions often make opportunities. The occasion of
          sickness may give opportunity for reflection.
          [1913 Webster]

WordNet (r) 3.0 (2006):

opportunity
    n 1: a possibility due to a favorable combination of
         circumstances; "the holiday gave us the opportunity to
         visit Washington"; "now is your chance" [syn:
         opportunity, chance]

The Devil's Dictionary (1881-1906):

OPPORTUNITY, n.  A favorable occasion for grasping a disappointment.