The substance that have sour taste, turns blue litmus red, liberates hydrogen with some metals and neutralises base is termed as an acid.
Acids react with metals to form hydrogen gas and a salt.
According to Arrhenius, acid is a compound that can release a proton or hydrogen ion [H+] in water.
According to Bronsted-Lowry concept, acid is defined as any hydrogen containing material which can donate a proton to other substances.
The pH of a solution is less than 7, the solution is called acidic.