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, including catalysis and complex formation. Lewis acids are often metal ions or molecules with an incomplete octet. Common examples of Lewis acids include aluminum chloride and boron trifluoride. These substances play a crucial role in organic chemistry, particularly in reactions like Friedel-Crafts acylation and alkylation, where they help facilitate the formation of new bonds by accepting electrons from other reactants.