How is "hydronium ion" defined?

A hydronium ion is a positively charged ion with the formula H3O+. It is formed from the reaction of water with a positive hydrogen ion, which consists of a single proton. The proton binds to one of the unshared electron pairs of the oxygen atom in the molecule of water.

The reaction of water with a proton is also called the protonation of water. Here, it is considered that water acts like a base. H3O+ is also known as the conjugate acid of water. It is formed whenever an acid, which has the ability to donate a proton, is dissolved in water. This mechanism can be represented as HA ? H+ + A-, where A- is the associated negative ion of the acid.