The Collaborative International Dictionary of English v.0.48:

Anion \An"i*on\, n. [Gr. ?, neut. ?, p. pr. of ? to go up; ? up
   + ? to go.] (Chem.)
   An electro-negative atom or group of atoms, or the element
   which, in electro-chemical decompositions, is evolved at the
   anode; -- opposed to cation. --Faraday.
   [1913 Webster]

WordNet (r) 3.0 (2006):

anion
    n 1: a negatively charged ion

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "anion":
   acid, acidity, agent, alkali, alkalinity, alloisomer, antacid,
   atom, base, biochemical, cation, chemical, chemical element,
   chromoisomer, compound, copolymer, dimer, electrocoating,
   electroetching, electrogalvanization, electrogilding,
   electrograving, electrolysis, electrolyte, electroplating, element,
   galvanization, heavy chemicals, high polymer, homopolymer,
   hydracid, inorganic chemical, ion, ionization, ionogen, isomer,
   macromolecule, metamer, molecule, monomer, neutralizer, nonacid,
   nonelectrolyte, organic chemical, oxyacid, polymer, pseudoisomer,
   radical, reagent, sulfacid, trimer