Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

anion
     n : a negatively charged ion