A Brønsted-Lowry acid is a substance that donates a proton (H⁺) to another substance in a chemical reaction. This definition expands the concept of acids beyond just those that produce hydrogen ions in water, allowing for a broader understanding of acid-base reactions in various environments. In this theory, when a Brønsted-Lowry acid donates a proton, it transforms into its corresponding Brønsted-Lowry base, which is the species that remains after the proton is lost. This interaction is fundamental in many chemical processes, including those in biological systems and industrial applications.