A Lewis acid is a chemical species that can accept an electron pair from a Lewis base during a chemical reaction. This interaction forms a coordinate covalent bond, which is essential in many chemical processes. Lewis acids are often electron-deficient and can include metal cations, certain molecules, and even some organic compounds. Common examples of Lewis acids include aluminum chloride (AlCl3) and boron trifluoride (BF3). These substances play a crucial role in various reactions, such as Friedel-Crafts acylation and alkylation, where they help facilitate the formation of new bonds by accepting electrons from other reactants.