An acid is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion activity greater than in pure water, i.e. a pH less than 7.0. That approximates the modern definition of Johannes Nicolaus Brønsted and Martin Lowry, who independently defined an acid as a compound which donates a hydrogen ion (H+) to another compound (called a base). Common examples include acetic acid (in vinegar) and sulfuric acid (used in car batteries). Acids can occur in solid, liquid or gaseous form, depending on the temperature. They can exist as pure substances or in solution.
Chemicals or substances having the property of an acid are said to be acidic.
For more, visit the Wikipedia entry for acid, URL: http://en.wikipedia.org/wiki/Acid