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A substance defined as an acid is one that produces hydrogen ions (H⁺) when dissolved in water. This definition aligns with the Arrhenius theory of acids, which states that acids increase the concentration of hydrogen ions in aqueous solutions.

When acids dissociate in water, they release protons (essentially H⁺ ions), which is a fundamental characteristic that allows them to exhibit their acidic properties, such as increasing the acidity of a solution, reacting with bases, and participating in various chemical reactions. This is why the definition related to hydrogen ion production is the most accurate in the context of acidity.

The other definitions, while relevant to other branches of acid-base theory—like the Bronsted-Lowry definition which describes acids as proton donors—do not specifically pertain to the basic, classical understanding of what acids do in solution.