From AMS Glossary
A positively charged species of chemical, symbol H+, the ionized form of the hydrogen atom.
The hydrogen ion is hydrated in aqueous solutions and is usually written as H3O+. Neutral water (pH 7) contains hydrogen ions at a concentration of 10-7 mol L-1. Dissolution of acids in water leads to an increase in the hydrogen ion concentration, and thus a decrease in the PH.