酒精度的定义是指
酒中纯乙醇(酒精)所含的容量百分比。
比如某酒100毫升中纯乙醇含量为10毫升，
这种酒的酒度就是10度。
英语中可以表达为
Alcohol by volume
wikipedia
Alcohol by volume (abbreviated as ABV, abv, or alc/vol) is a standard measure of how much alcohol (ethanol) is contained in a given volume of an alcoholic beverage (expressed as a volume percent).
酒精体积(简写为ABV、abv或alcvol)是衡量酒精饮料中酒精(乙醇)含量的标准方法(以体积百分比表示)。
It is defined as the number of millilitres (mL) of pure ethanol present in 100 mL (3.4 fl. oz) of solution at 20 °C (68 °F).
它是指在20摄氏度(68华氏度)下，100毫升(3.4盎司)的溶液中所含的纯乙醇的毫升数。
The ABV standard is used worldwide.
ABV标准在世界范围内使用。
例：
wine at 12% ABV
12度的酒
A：What's the alcohol content of QingDao beer?
青岛啤酒的麦芽汁浓度是多少？
（中文里“啤酒度数”一般指麦芽汁浓度，并不是酒精占比）
B：It's about 7%.
大概百分之七。
alcohol content
（酒精容量）
另外，国外还有另一种常见的计量单位
proof
If you get a bottle of vodka from your local liquor store, you’ll probably see two different numbers telling you how much alcohol it contains. The first is an alcohol by volume (ABV) percentage, which is relatively intuitive to understand: it’s the percentage of alcohol in the overall liquid and is pretty standard internationally.
如果你从当地的酒类商店买了一瓶伏特加，你可能会看到两个不同的数字告诉你酒里含有多少酒精。第一个是酒精体积百分比(ABV)，这是比较直观的理解:它是酒精在整个液体中的百分比，在国际上是相当标准的。
The other measurement is proof, a measurement of alcohol content that varies from country to country. The proof measurement harks back to 16th-century England, when the government would put an extra tax on “proof spirits”—that is, liquor that contained a certain higher amount of alcohol.
还有一种就是酒精浓度，各国的测量方法各不相同。这种酒精含量的测量方式可以追溯到16世纪的英国，当时政府会对达到酒精浓度标准的，即酒精含量较高的烈性酒征收额外的税。
A pellet of gunpowder was soaked in spirits. If the pellet burned, the spirits were taxed at a higher rate. The burning was proof of its alcohol content (hence the term 'proof' to measure alcohol content).
方法就是将一粒火药浸在烈性酒里。如果这粒火药燃烧了，就证明酒为烈性酒，并将被征收更高的税。燃烧是酒精含量的证明(因此有了测量酒精含量的“证明”一词)。
注：啤酒的度数是指发酵液中麦芽糖的浓度，不是指成品酒中乙醇的含量，而葡萄酒的酒精度主要由葡萄果实中的含糖量决定。所以都和白酒的酒精度数不同。
