Source : Webster's Revised Unabridged Dictionary (1913)

Gallic \Gal"lic\ (277), a. [From {Gall} the excrescence.]
   Pertaining to, or derived from, galls, nutgalls, and the
   like.

   {Gallic acid} (Chem.), an organic acid, very widely
      distributed in the vegetable kingdom, being found in the
      free state in galls, tea, etc., and produced artificially.
      It is a white, crystalline substance, {C6H2(HO)3.CO2H},
      with an astringent taste, and is a strong reducing agent,
      as employed in photography. It is usually prepared from
      tannin, and both give a dark color with iron salts,
      forming tannate and gallate of iron, which are the
      essential ingredients of common black ink.

Source : WordNet®

gallic acid
     n : a colorless crystalline acid obtained from tannin