[1] : The Collaborative International Dictionary of English v.0.48

occasion \occa"sion\ o^kk=a"zhu^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.
      1913 webster

            the unlooked-for incidents of family history, and
            its hidden excitements, and its arduous occasions.
                                                  --i. taylor.
      1913 webster

   2. a favorable opportunity; a convenient or timely chance;
      convenience.
      1913 webster

            sin, taking occasion by the commandment, deceived
            me.                                   --rom. vii.
                                                  11.
      1913 webster

            i'll take the occasion which he gives to bring
            him to his death.                     --waller.
      1913 webster

   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.
      1913 webster

            her beauty was the occasion of the war. --dryden.
      1913 webster

   4. need; exigency; requirement; necessity; as, i have no
      occasion for firearms.
      1913 webster

            after we have served ourselves and our own
            occasions.                            --jer. taylor.
      1913 webster

            when my occasions took me into france. --burke.
      1913 webster

   5. a reason or excuse; a motive; a persuasion.
      1913 webster

            whose manner was, all passengers to stay,
            and entertain with her occasions sly. --spenser.
      1913 webster

   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.
        1913 webster
see also:
ob- chance occident on occasion opportunity 

[2] : The Collaborative International Dictionary of English v.0.48

occasion \occa"sion\ o^kk=a"zhu^n, v. t. imp.  p. p.
   occasioned o^kk=a"zhu^nd; p. pr.  vb. n.
   occasioning. cf. f. occasionner.
   to give occasion to; to cause; to produce; to induce; as, to
   occasion anxiety. --south.
   1913 webster

         if we inquire what it is that occasions men to make
         several combinations of simple ideas into distinct
         modes.                                   --locke.
   1913 webster
see also:
occasioned occasioning 

[3] : WordNet (r) 2.0

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
     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,
         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 : give occasion to
see also:
juncture affair social occasion function social function 

[4] : Moby Thesaurus II by Grady Ward, 1.0

153 moby thesaurus words for "occasion":
   a leg up, advantage, adventure, affair, antecedent, antecedents,
   author, bare necessities, base, basis, bear, beget, break, breed,
   bring about, bring forth, bring on, bring to effect, bring to pass,
   call, call for, call forth, call up, causation, cause,
   cause and effect, celebration, ceremony, chance, circumstance,
   clear stage, commemoration, conceive, condition, conjuncture,
   contingency, create, demand, demand for, desideration, desideratum,
   determinant, determinative, do, effect, effectuate, element,
   elicit, engender, episode, essential, essentials, establish,
   etiology, event, eventuality, evoke, excuse, experience, fact,
   factor, fair field, fair game, father, found, foundation, function,
   gala, generate, gestate, give birth to, give occasion to,
   give origin to, give rise to, go, ground, grounds, hap, happening,
   happenstance, impel, impulse, inaugurate, incident, incitement,
   indispensable, induce, inducement, institute, juncture,
   justification, liberty, look-in, make, make for, matter of fact,
   milestone, moment, must, must item, necessaries, necessities,
   necessity, need, need for, obligation, observance, occasionally,
   occurrence, on occasion, opening, opportunism, opportunity,
   originate, particular, party, phenomenon, place, prerequirement,
   prerequisite, principle, produce, prompt, prompting, provocation,
   provoke, reality, realize, reason, requirement, requisite,
   requisition, right, room, scope, set afloat, set on foot, set up,
   shot, show, sire, squeak, stepping-stone, stimulus, the necessary,
   the needful, thing, time, turn of events, want, warrant, work,
   work up

Occasion >> O Words
Occasion, definition of term: Occasion
occasion_pag1.html

On Occasion >> O Words
On Occasion, definition of term: On Occasion
on+occasion_pag1.html