In an acid-base reaction according to the Arrhenius definition, an acid donates H+ ions, while a base donates OH- ions in an aqueous solution. The H+ ions from the acid combine with the OH- ions from the base to form water (H2O).

For example, when hydrochloric acid (HCl) and sodium hydroxide (NaOH) are mixed together, the HCl donates H+ ions and the NaOH donates OH- ions. The H+ and OH- ions react to form water molecules:

HCl + NaOH → NaCl + H2O

This reaction is often referred to as a neutralization reaction because the acid and base neutralize each other to form a salt (in this case, sodium chloride) and water.

The Arrhenius definition of acids and bases is still widely used today, especially in introductory chemistry courses. However, there are other more modern definitions of acids and bases, such as the Bronsted-Lowry definition and the Lewis definition.