Olive oil is more complicated than many people think. It's not just a matter of going down to the grocery store and picking up any old bottle of oil. There is a plethora of different classifications and tastes. It takes a keen tongue and a strong mind to classify oils in this sometimes confusing field properly.
Extra virgin olive oil is the crème de la crème of oil. It is the best olive oil in the world with perfect purity and zero defects.
A virgin is thought of as something that is uncorrupted. Extra virgin is purer than pure with zero defects and a flavor of freshly harvested olives.
It is the most expensive of the oils and this classification must undergo a rigorous chemical and oral test by the International Olive Council. Only the elite oils can ever hope to ascend to these lofty heights.
Chemical Make-Up of Extra Virgin Olive Oil
This is the best olive oil in the world and therefore requires rigorous chemical testing in order to verify its authenticity as an extra virgin oil. It must have free acidity and have no more than 0.8 grams per 100 grams of oleic acid. It must also have a below 20 milliequivalent O2 level.
Chemical tests are also conducted to make sure that it doesn't degrade whilst under 86 degrees Fahrenheit. On a side note, any oil that isn't produced entirely through mechanical means is instantly disqualified. This is to reduce the chance of having a quality chasm between different batches of oil, which always comes because of human error.