Source : Webster's Revised Unabridged Dictionary (1913)

Joke \Joke\, n. [L. jocus. Cf {Jeopardy}, {Jocular}, {Juggler}.]
   1. Something said for the sake of exciting a laugh; something
      witty or sportive (commonly indicating more of hilarity or
      humor than jest); a jest; a witticism; as, to crack
      good-natured jokes.

            And gentle dullness ever loves a joke. --Pope.

            Or witty joke our airy senses moves To pleasant
            laughter.                             --Gay.

   2. Something not said seriously, or not actually meant;
      something done in sport.

            Inclose whole downs in walls, 't is all a joke.
                                                  --Pope.

   {In joke}, in jest; sportively; not meant seriously.

   {Practical joke}. See under {Practical}.

Practical \Prac"ti*cal\, a. [L. practicus active, Gr. ? fit for
   doing or performing, practical, active, fr. ? to do, work,
   effect: cf. F. pratique, formerly also practique. Cf.
   {Pragmatic}, {Practice}.]
   1. Of or pertaining to practice or action.

   2. Capable of being turned to use or account; useful, in
      distinction from {ideal} or {theoretical}; as, practical
      chemistry. ``Man's practical understanding.'' --South.
      ``For all practical purposes.'' --Macaulay.

   3. Evincing practice or skill; capable of applying knowledge
      to some useful end; as, a practical man; a practical mind.

   4. Derived from practice; as, practical skill.

   {Practical joke}, a joke put in practice; a joke the fun of
      which consists in something done, in distinction from
      something said; esp., a trick played upon a person.

Source : WordNet®

practical joke
     n : a prank or trick played on a person (especially one intended
         to make the victim appear foolish)