Most people think a unit is a half-pint of beer or one glass of wine - but, in fact, a half-pint represents 1.2 units and a 175 ml glass contains 2.1 units. Even using a smaller glass of 125 ml doesn't quite do it, either, even though this is the advice from the UK's Department of Health. This is only true if the alcoholic content is 8%; but as most wines have a 12% content, the smaller glass would still represent 1.5 units.
Then there's the definition of alcohol itself, and this is to do with its ethanol content. The standard ethanol level worldwide is 10 to 15 g, yet it's just 8 g in the UK. So when groups such as the World Cancer Research Fund reported recently on recommended safe levels, they had in mind alcohol with a 15 g ethanol level.
It's enough to drive anyone to drink.
(Source: British Medical Journal, 2007; 335: 1008).