A Lewis acid is a chemical species that can accept an electron pair from a Lewis base during a chemical reaction. This definition broadens the concept of acids beyond traditional Brønsted-Lowry acids, which donate protons. Lewis acids are often metal ions or molecules with an incomplete octet, making them capable of forming coordinate covalent bonds. Common examples of Lewis acids include aluminum chloride and boron trifluoride. These substances play a crucial role in various chemical reactions, including catalysis and organic synthesis, by facilitating the formation of new bonds and enhancing reaction rates.