Acids and bases are fundamental chemical concepts defined by how they behave in solution. The Arrhenius definition states that acids produce H+ ions and bases produce OH- ions in water. A broader view is the Brønsted-Lowry definition, where an acid is a proton (H+) donor and a base is a proton acceptor. The most inclusive is the Lewis definition, identifying acids as electron-pair acceptors and bases as electron-pair donors. These definitions are crucial for understanding reactions, pH calculations, and buffer systems.