In chemistry, a Lewis acid is any acid that can accept a pair of electrons and form a coordinate covalent bond, after the American chemist Gilbert Lewis. The term acid is ambiguous; this is one of the possible interpretations.

An electrophile is a Lewis acid. A Lewis acid usually has an empty LUMO.

Some common Lewis acids include aluminium chloride, iron(III) chloride, boron trifluoride and ytterbium(III) triflate .

See also

• Lewis base
• Acid
• Base
• Acid-base reaction theories