Definition of "hydronium"

The case of hydronium ions (H3O+) is an example of this. In the gase[sic] phase, hydronium is a weak acid. The data in Table 3.10 indicates that bare hydronium ions have a higher electron affinity than most metal surfaces, so a bare hydronium ion should be neutralized when it absorbs on a bare metal surface. However, a hydrated hydronium ion is a stronger acid.

1996, Richard I. Masel, Principles of Adsorption and Reaction on Solid Surfaces, Wiley, page 148

Acids are compounds that produce hydronium ions when mixed with water. […] The hydronium ion is a very special ion because it is the defining particle of all acids.

2006, Johanna Holm, Forgotten Chemistry: A Refresher Course, Barron's Educational Series, page 160

Hydronium is not a way of stabilizing an acid, because the structure itself is so quickly transient that it can be considered as not existing.

2016, Philippe Deprez, Textbook of Chemical Peels: Superficial, Medium, and Deep Peels in Cosmetic Practice, 2nd edition, Taylor & Francis (CRC Press), page 48

There is experimental evidence for the formation of coordination groups of hydroniums with several water molecules by hydrogen bridges.

2016, Andrés G. Muñoz, Photoelectrochemical Solar Conversion Systems: Molecular and Electronic Aspects‎, Taylor & Francis (CRC Press), page 206