The Collaborative International Dictionary of English v.0.48:

Tartar \Tar"tar\, n. [F. tartre (cf. Pr. tartari, Sp., Pg., &
   It. tartaro, LL. tartarum, LGr. ?); perhaps of Arabic
   origin.]
   1. (Chem.) A reddish crust or sediment in wine casks,
      consisting essentially of crude cream of tartar, and used
      in marking pure cream of tartar, tartaric acid, potassium
      carbonate, black flux, etc., and, in dyeing, as a mordant
      for woolen goods; -- called also argol, wine stone,
      etc.
      [1913 Webster]

   2. A correction which often incrusts the teeth, consisting of
      salivary mucus, animal matter, and phosphate of lime.
      [1913 Webster]

   Cream of tartar. (Chem.) See under Cream.

   Tartar emetic (Med. Chem.), a double tartrate of potassium
      and basic antimony. It is a poisonous white crystalline
      substance having a sweetish metallic taste, and used in
      medicine as a sudorific and emetic.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tartar emetic
    n 1: a poisonous colorless salt used as a mordant and in
         medicine [syn: tartar emetic, antimony potassium
         tartrate]