A hydrogen ion refers to the nucleus of a hydrogen ion isolated from its electron. The term is also commonly used to refer to the hydrogen ion in water solutions.
The nucleus of a hydrogen atom is made up of a positively charged particle called a proton. Therefore, the hydrogen ion, whose symbol is H+, represents a proton. Since the isolated nucleus of hydrogen can easily combine with other particles, the isolated hydrogen ion can only be present in an almost particle-free space or in gaseous state. Moreover, the amount of hydrogen ion in a water solution is used to calculate the level of acidity.