The Collaborative International Dictionary of English v.0.48:

Gallic \Gal"lic\ (277), a. [From Gall the excrescence.]
   Pertaining to, or derived from, galls, nutgalls, and the
   like.
   [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

gallic acid
    n 1: a colorless crystalline acid obtained from tannin