The Collaborative International Dictionary of English v.0.48:

Cation \Cat"i*on\, n. [Gr. ? downward + ? going, p. pr. of ? to
   go.] (Chem.)
   a positively charged atom, radical, or molecule, which in
   electrolysis migrates to the cathode; a positive ion; --
   opposed to anion. --Faraday.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

cation
    n 1: a positively charged ion

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "cation":
   acid, acidity, agent, alkali, alkalinity, alloisomer, anion,
   antacid, atom, base, biochemical, 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