Acid (noun)

1. A sour substance.
2. A chemical compound that typically contains oxygen and has a pH less than 7, being able to donate a proton (H+) to a base, and able to form a covalent bond with an electron pair, known as Bronsted-Lowry acid-base theory.

Origin:

Middle english, from latin acidus, from acēre to be sour; akin to old english ǣsce sour.

Examples:

1. Lemon juice is an example of an acid.
2. Acids are commonly used in laboratory experiments.
3. The acid in your stomach helps to break down food.
4. Acid rain is caused by the release of sulfur dioxide and nitrogen oxide into the atmosphere.
5. Acid can be used to etch metal.