Hydrogen ions are positively charged particles made up of hydrogen atoms. They don't have any electrons and are responsible for making a substance acidic when they're present. Full definition
The concentration of hydrogen ions in the liquid is measured by the pH scale, and then it is expressed on a scale of 0 - 14. (selfcarer.com)
This increase in the ocean's hydrogen ion concentration corresponds to a reduction in pH, or an increase in acidity. (metlink.org)
These operations were conducted at numerous places, and, during the beam extraction, we investigated how the flow of negative hydrogen ions changed. (sciencedaily.com)