A conjugate base is the species that remains after an acid donates a proton (H⁺) during a chemical reaction. When an acid loses a proton, it transforms into its conjugate base, which can potentially accept a proton in the future. This concept is essential in understanding acid-base reactions and the behavior of acids and bases in various chemical environments. For example, when hydrochloric acid (HCl) donates a proton, it becomes its conjugate base, chloride ion (Cl⁻). The relationship between acids and their conjugate bases is a fundamental aspect of the Brønsted-Lowry theory of acids and bases, which emphasizes the transfer of protons in chemical reactions.