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A Bronsted-Lowry acid is defined as a substance that donates a proton (H⁺) in a chemical reaction. This principle is fundamental in acid-base chemistry, distinguishing it from the Arrhenius definition, which focuses solely on the production of hydrogen ions in water.

When a Bronsted-Lowry acid donates a proton, it becomes a conjugate base, and this process is key in many chemical reactions, particularly those involving acid-base equilibria. The acceptance or donation of protons is central to understanding various chemical processes, including reaction mechanisms, buffer systems, and physiological functions.

For example, when hydrochloric acid (HCl) dissolves in water, it donates a proton to water, forming hydronium ions (H₃O⁺) and chloride ions (Cl⁻). This illustrates the proton donation characteristic of Bronsted-Lowry acids. Understanding this definition allows for a clearer conceptualization of acid-base reactions beyond the scope of simple hydrogen ion production.