The Lewis definitions refer to a set of concepts in acid-base chemistry proposed by Gilbert N. Lewis. According to these definitions, an acid is a substance that can accept an electron pair, while a base is a substance that can donate an electron pair. This approach broadens the understanding of acids and bases beyond the traditional Brønsted-Lowry definitions, which focus on proton transfer. The Lewis acid-base theory is useful in explaining various chemical reactions, particularly in coordination chemistry and organic reactions. It highlights the role of electron pairs in chemical bonding and reactivity, making it a fundamental concept in modern chemistry.