A Brønsted-Lowry base is defined as a substance that can accept protons (H⁺ ions) in a chemical reaction. This concept is part of the broader Brønsted-Lowry theory of acids and bases, which emphasizes the transfer of protons between species. When a base accepts a proton, it often forms a conjugate acid, which can then donate the proton in a subsequent reaction. In contrast to the Arrhenius definition of bases, which focuses on hydroxide ions (OH⁻), the Brønsted-Lowry definition allows for a wider range of substances to be classified as bases. This includes compounds like ammonia (NH₃), which can accept a proton to form ammonium (NH₄⁺).