The Collaborative International Dictionary of English v.0.48:

Fast \Fast\, v. i. [imp. & p. p. Fasted; p. pr. & vb. n.
   Fasting.] [AS. f[ae]stan; akin to D. vasten, OHG.
   fast[=e]n, G. fasten, Icel. & Sw. fasta, Dan. faste, Goth.
   fastan to keep, observe, fast, and prob. to E. fast firm.]
   1. To abstain from food; to omit to take nourishment in whole
      or in part; to go hungry.
      [1913 Webster]

            Fasting he went to sleep, and fasting waked.
                                                  --Milton.
      [1913 Webster]

   2. To practice abstinence as a religious exercise or duty; to
      abstain from food voluntarily for a time, for the
      mortification of the body or appetites, or as a token of
      grief, or humiliation and penitence.
      [1913 Webster]

            Thou didst fast and weep for the child. --2 Sam.
                                                  xii. 21.
      [1913 Webster]

   Fasting day, a fast day; a day of fasting.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fasting
    n 1: abstaining from food [syn: fast, fasting]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "fasting":
   Albigensianism, Catharism, Day of Atonement, Franciscanism, Lenten,
   Sabbatarianism, Trappism, Waldensianism, Yoga, Yom Kippur,
   abstinence, anchoritic monasticism, anchoritism, asceticism,
   austerity, cold purgatorial fires, dog-hungry, empty, eremitism,
   famished, famishing, flagellation, hair shirt, half-famished,
   half-starved, hungering, hungry, lustration, maceration,
   mendicantism, monachism, monasticism, mortification, peckish,
   penance, penitence, penitential act, penitential exercise,
   pinched with hunger, purgation, purgatory, puritanism,
   quadragesimal, ravening, ravenous, repentance, rigor,
   sackcloth and ashes, self-denial, self-mortification, sharp-set,
   starved, starving, uneating, unfed, unfilled, voluntary poverty,
   voracious, wolfish