The Collaborative International Dictionary of English v.0.48:

Sherbet \Sher"bet\, n. [Ar. sherbet, shorbet, sharbat, properly,
   one drink or sip, a draught, beverage, from shariba to drink.
   Cf. Sorbet, Sirup, Shrub a drink.]
   1. A refreshing drink, common in the East, made of the juice
      of some fruit, diluted, sweetened, and flavored in various
      ways; as, orange sherbet; lemon sherbet; raspberry
      sherbet, etc.
      [1913 Webster]

   2. A flavored water ice.
      [1913 Webster]

   3. A preparation of bicarbonate of soda, tartaric acid,
      sugar, etc., variously flavored, for making an
      effervescing drink; -- called also sherbet powder.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sherbet
    n 1: a frozen dessert made primarily of fruit juice and sugar,
         but also containing milk or egg-white or gelatin [syn:
         sherbert, sherbet]