hydrogen ion

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The positively charged ion of hydrogen, H⁺, formed by removal of the electron from atomic hydrogen and found in all aqueous solutions of acids.

Wiktionary

1. n. chemistry Used other than as an idiom: see hydrogen,‎ ion. ; H- or H+
2. n. chemistry 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. n. chemistry a bare hydrogen nucleus; a proton , deuteron or triton

WordNet 3.0

1. n. a positively charged atom of hydrogen; that is to say, a normal hydrogen atomic nucleus