Source : Webster's Revised Unabridged Dictionary (1913)

Gratuity \Gra*tu"i*ty\, n.; pl. {Gtratuities}. [F. gratuit['e],
   or LL. gratuitas.]
   1. Something given freely or without recompense; a free gift;
      a present. --Swift.

   2. Something voluntarily given in return for a favor or
      service, as a recompense or acknowledgment.

Source : WordNet®

gratuity
     n 1: a relatively small amount of money given for services
          rendered (as by a waiter) [syn: {tip}, {pourboire}, {baksheesh},
           {bakshish}, {bakshis}, {backsheesh}]
     2: an award (as for meritorious service) given without claim or
        obligation