What are antioxidants?
An antioxidant is a chemical that take the place of an oxidant atom in a molecule. Some chemicals are “starved” for oxygen. These are called Free Radicals. When such a chemical enters the body, it can start a chain reaction that destroys and mutates cells. Free radicals are believed to be a major cause for cancer and other diseases. When a body is attacked by a large number of free radicals it is under oxidative stress.

Antioxidants are the foods that relieve oxidative stress by taking the place of oxygen with these free radicals. Thus, the free radicals are absorbed, and are no longer a danger to the body.

Among the foods known as powerful antioxidants are vitamin C, vitamin E, some enzymes, some berries, and chocolate.
The potency of antioxidants is measured by their capacity to absorb free radicals. This measure is called Oxygen Radical Absorbance Capacity (ORAC). Quantitatively, the ORAC value of an antioxidant is expressed in comparison to vitamin E (trolox). It is generally believed that an average adult in the US needs to consume about 5,000 ORAC units a day to combat oxidative stress.

The US Department of Agriculture has published a report detailing the ORAC values of various foods. They are expressed per 100 grams (about 3.5 Ounces). Some of these are:
Kidney Beans 8,606
Pecans 17,940
Tomatoes 546
Cranberries 9,090
Goji Berries 3,290
Dark Chocolate 20,816

As can be seen, dark chocolate is one of the top antioxidant foods.