Acid

An acid is something that has a pH less than 7.0 .
Sometimes, the abbreviation HA is used to mean an acid.
The chemistry definition of an acid is: "a substance that donates a hydrogen ion (H⁺) to another substance, which is called a base. (Or give out hydrogen ions when dissolved in water.)"
Acids can be commonly found in nature. Some of these include:

• Vinegar is probably one of the oldest known acids; it contains acetic acid.
• Nitric acid, NHO₃ is known since about the 13th century.
• Citric acids, C₆H₈O₇ can be found in many kinds of fruit. They were probably discovered by Geber in the 8th century.^[1][2][3]
• Lactic acid was found by Carl Wilhelm Scheele in 1780. It can be found in sour milk products, such as Yogurt.
• Sulfuric acid, H₂SO₄ was probably discovered by Geber. Today it can be found in batteries amongst others.
• Hydrochloric acid, HCl

Some acids are strong and others are weak. The weak acids hold on to some of their protons, while the strong acids let go of all of them. Weak acids generally have a pH value of 4-6 while strong acids have a pH value of 1-3.

source : wikipedia