A Brønsted-Lowry acid is a substance that donates a proton (H⁺) to another substance in a chemical reaction. This definition is part of the Brønsted-Lowry theory, which focuses on the transfer of protons rather than just the presence of hydrogen ions. In this theory, when a Brønsted-Lowry acid donates a proton, it becomes a Brønsted-Lowry base, which is a substance that accepts a proton. This interaction is fundamental in many chemical reactions, including those in biological systems and industrial processes.