A hydrogen ion - the cation derived from a hydrogen atom by the removal of the electron. The concentration, or more correctly, the activity of protons in a solution is quantified by pH, the negative decadic logarithm of the molar activity of the hydrogen ions in the solution.

Proton can be used to refer specifically to the cation derived from protium, the most abundant isotope of hydrogen (with nucleon number unity), or more generally to the cation derived from any atom of naturally occurring hydrogen, of which a small proportion is deuterium.

The equivalent name for the cation derived from a heavier isotope of hydrogen would be a deuteron (deuterium) or a triton (tritium).