The Collaborative International Dictionary of English v.0.48:

Fast \Fast\, n. [OE. faste, fast; cf. AS. f[ae]sten, OHG. fasta,
   G. faste. See Fast, v. i.]
   1. Abstinence from food; omission to take nourishment.
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            Surfeit is the father of much fast.   --Shak.
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   2. Voluntary abstinence from food, for a space of time, as a
      spiritual discipline, or as a token of religious
      humiliation.
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   3. A time of fasting, whether a day, week, or longer time; a
      period of abstinence from food or certain kinds of food;
      as, an annual fast.
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   Fast day, a day appointed for fasting, humiliation, and
      religious offices as a means of invoking the favor of God.


   To break one's fast, to put an end to a period of
      abstinence by taking food; especially, to take one's
      morning meal; to breakfast. --Shak.
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WordNet (r) 3.0 (2006):

fast day
    n 1: a day designated for fasting