The Collaborative International Dictionary of English v.0.48:

Occasion \Oc*ca"sion\ ([o^]k*k[=a]"zh[u^]n), v. t. [imp. & p. p.
   Occasioned ([o^]k*k[=a]"zh[u^]nd); p. pr. & vb. n.
   Occasioning.] [Cf. F. occasionner.]
   To give occasion to; to cause; to produce; to induce; as, to
   occasion anxiety. --South.
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         If we inquire what it is that occasions men to make
         several combinations of simple ideas into distinct
         modes.                                   --Locke.
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The Collaborative International Dictionary of English v.0.48:

Occasion \Oc*ca"sion\ ([o^]k*k[=a]"zh[u^]n), n. [F. occasion, L.
   occasio, fr. occidere, occasum, to fall down; ob (see Ob-)
   + cadere to fall. See Chance, and cf. Occident.]
   1. A falling out, happening, or coming to pass; hence, that
      which falls out or happens; occurrence; incident; event.
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            The unlooked-for incidents of family history, and
            its hidden excitements, and its arduous occasions.
                                                  --I. Taylor.
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   2. A favorable opportunity; a convenient or timely chance;
      convenience.
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            Sin, taking occasion by the commandment, deceived
            me.                                   --Rom. vii.
                                                  11.
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            I'll take the occasion which he gives to bring
            Him to his death.                     --Waller.
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   3. An occurrence or condition of affairs which brings with it
      some unlooked-for event; that which incidentally brings to
      pass an event, without being its efficient cause or
      sufficient reason; accidental or incidental cause.
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            Her beauty was the occasion of the war. --Dryden.
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   4. Need; exigency; requirement; necessity; as, I have no
      occasion for firearms.
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            After we have served ourselves and our own
            occasions.                            --Jer. Taylor.
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            When my occasions took me into France. --Burke.
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   5. A reason or excuse; a motive; a persuasion.
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            Whose manner was, all passengers to stay,
            And entertain with her occasions sly. --Spenser.
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   On occasion,
      (a) in case of need; in necessity; as convenience
          requires. "That we might have intelligence from him on
          occasion," --De Foe.
      (b) occasionally; from time to time; now and then.
          [1913 Webster +PJC]

   Syn: Need; incident; use. See Opportunity.
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WordNet (r) 3.0 (2006):

occasion
    n 1: an event that occurs at a critical time; "at such junctures
         he always had an impulse to leave"; "it was needed only on
         special occasions" [syn: juncture, occasion]
    2: a vaguely specified social event; "the party was quite an
       affair"; "an occasion arranged to honor the president"; "a
       seemingly endless round of social functions" [syn: affair,
       occasion, social occasion, function, social function]
    3: reason; "there was no occasion for complaint"
    4: the time of a particular event; "on the occasion of his 60th
       birthday"
    5: an opportunity to do something; "there was never an occasion
       for her to demonstrate her skill"
    v 1: give occasion to