hydrogen ion

Short definition from Wiktionary

Noun

1. 1
   Used other than figuratively or idiomatically: see hydrogen, ion.: H⁻ or H⁺
2. 2
   a proton combined with one or more water molecules; usually written H₃O⁺ and called the hydronium ion though is best considered as H₉O₄⁺ but is often written H⁺_((aq)) for simplicity
3. 3
   a bare hydrogen nucleus; a proton, deuteron or triton