The Collaborative International Dictionary of English v.0.48:

Tickle \Tic"kle\, v. t. [imp. & p. p. Tickled; p. pr. & vb. n.
   Tickling.] [Perhaps freq. of tick to beat; pat; but cf.
   also AS. citelian to tickle, D. kittelen, G. kitzlen, OHG.
   chizzil[=o]n, chuzzil[=o]n, Icel. kitla. Cf. Kittle, v. t.]
   1. To touch lightly, so as to produce a peculiar thrilling
      sensation, which commonly causes laughter, and a kind of
      spasm which become dangerous if too long protracted.
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            If you tickle us, do we not laugh?    --Shak.
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   2. To please; to gratify; to make joyous.
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            Pleased with a rattle, tickled with a straw. --Pope.
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            Such a nature
            Tickled with good success, disdains the shadow
            Which he treads on at noon.           --Shak.
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WordNet (r) 3.0 (2006):

tickling
    adj 1: exciting by touching lightly so as to cause laughter or
           twitching movements [syn: tickling, tingling,
           titillating]
    n 1: the act of tickling [syn: tickle, tickling,
         titillation]