The Brønsted-Lowry acid-base theory defines acids as substances that donate protons (H⁺ ions) and bases as substances that accept protons. This theory expands the traditional definition of acids and bases beyond just their behavior in water, allowing for a broader understanding of acid-base reactions in various environments. In this theory, when an acid donates a proton, it forms its conjugate base, while the base that accepts the proton forms its conjugate acid. This relationship helps to explain the dynamic nature of acid-base reactions, where substances can act as either acids or bases depending on the context of the reaction.