The Collaborative International Dictionary of English v.0.48:

Purpose \Pur"pose\, n. [OF. purpos, pourpos, propos, L.
   propositum. See Propound.]
   1. That which a person sets before himself as an object to be
      reached or accomplished; the end or aim to which the view
      is directed in any plan, measure, or exertion; view; aim;
      design; intention; plan.
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            He will his firste purpos modify.     --Chaucer.
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            As my eternal purpose hath decreed.   -- Milton.
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            The flighty purpose never is o'ertook
            Unless the deed go with it.           --Shak.
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   2. Proposal to another; discourse. [Obs.] --Spenser.
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   3. Instance; example. [Obs.] --L'Estrange.
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   In purpose, Of purpose, On purpose, with previous
      design; with the mind directed to that object;
      intentionally. On purpose is the form now generally used.
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   Syn: design; end; intention; aim. See Design.
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WordNet (r) 3.0 (2006):

on purpose
    adv 1: with intention; in an intentional manner; "he used that
           word intentionally"; "I did this by choice" [syn:
           intentionally, deliberately, designedly, on
           purpose, purposely, advisedly, by choice, by
           design] [ant: accidentally, by chance,
           circumstantially, unexpectedly, unintentionally]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "on purpose":
   advisedly, by design, calculatedly, consciously, contemplatedly,
   deliberately, designedly, in cold blood, intentionally, knowingly,
   meditatedly, pointedly, premeditatedly, purposefully, purposely,
   purposively, voluntarily, willfully, with full intent, with intent,
   with malice aforethought, with purpose, wittingly