H3O+

pH Scale: A measure of how acidic or basic water substances are. The scale ranges from 0-14; lower values indicate acidity while higher values indicate basicity.

Acid Dissociation Constant (Ka): A measure of the strength of an acid in solution; it's essentially the equilibrium constant for the dissociation reaction of an acid into its ions.

Proton Donor: In chemistry terms, this refers to acids because they donate protons (H+) to other molecules during chemical reactions.