https://doi.org/10.1351/goldbook.B00744
A molecular entity capable of donating a hydron (proton) to a base, (i.e. a 'hydron donor') or the corresponding chemical species. For example: H2O, H3O+, CH3CO2H, H2SO4, HSO4−, HCl, CH3OH, NH3.
See also: conjugate acid–base pair