The Collaborative International Dictionary of English v.0.48:

Tannic \Tan"nic\, a.
   Of or pertaining to tan; derived from, or resembling, tan;
   as, tannic acid.
   [1913 Webster]

   Tannic acid. (Chem.)
   (a) An acid obtained from nutgalls as a yellow amorphous
       substance, C14H10O9, having an astringent taste, and
       forming with ferric salts a bluish-black compound, which
       is the basis of common ink. Called also tannin, and
       gallotannic acid.
   (b) By extension, any one of a series of astringent
       substances resembling tannin proper, widely diffused
       through the vegetable kingdom, as in oak bark, willow,
       catechu, tea, coffee, etc.
       [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Tannin \Tan"nin\, n. [Cf. F. tannin.] (Chem.)
   Same as Tannic acid, under Tannic.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tannin
    n 1: any of various complex phenolic substances of plant origin;
         used in tanning and in medicine [syn: tannin, tannic
         acid]