Original gravity. The specific gravity (what you measure with your hydrometer) of the wort pre-fermentation. It is an indication of the amount of sugars in the wort and therefore the potential ABV. Actual ABV is calculated by comparing the OG to FG (final gravity, post fermentation) to see how many of those sugars actually got converted to alcohol.
You can't be a real country unless you have a beer and an airline. It helps if you have some kind of a football team, or some nuclear weapons, but at the very least you need a beer. -Frank Zappa