means An atom or molecule that has gained or lost one or more electrons, giving it a net positive or negative electric charge.

from From Greek ion, 'going,' the present participle of ienai, 'to go' — coined in the 1830s by the polymath William Whewell at the request of Michael Faraday, because these charged particles 'go' toward one electrode or the other. The same root gives us the anion (going up) and cation (going down), named for the directions they travel in solution.