Acetic acid, CH3COOH, also known as ethanoic acid, is an organic acid that gives vinegar its sour taste and pungent smell. It is a weak acid, in that it is only a partially dissociated acid in an aqueous solution. Pure, water-free acetic acid (glacial acetic acid) is a colorless liquid that absorbs water from the environment (hygroscopy), and freezes at 16.5 °C (62 °F) to a colorless crystalline solid. The pure acid and its concentrated solutions are dangerously corrosive.

Acetic acid is one of the simplest carboxylic acids. It is an important chemical reagent and industrial chemical, used in the production of polyethylene terephthalate mainly used in soft drink bottles; cellulose acetate, mainly for photographic film; and polyvinyl acetate for wood glue, as well as synthetic fibers and fabrics. In households, diluted acetic acid is often used in descaling agents. In the food industry, acetic acid is used under the food additive code E260 as an acidity regulator and as a condiment.