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 between molecules. 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 processes, including acid-base reactions, and helps explain the behavior of various acids and bases in different environments.