This is a question very much like "what's blue?" It depends very much on who's asking the question and how good the memory is of the person who answers it.
Most scientific studies on the health effects of alcohol have generally defined a "drink" as 8 to 12 grams of alcohol.
What does this mean to your consumption patterns?
-- A four-ounce glass of table wine (12 percent alcohol) has about 14 grams of alcohol.
-- A 12-ounce serving of American beer (3.2 percent alcohol) has about 11.5 grams of alcohol.
-- A mixed drink with a jigger (1.5 ounces) of 80-proof spirits has about 18 grams of alcohol.
As you'll read in other chapters of this book, there are significant differences among the health effects associated with these three basic types of beverage alcohol. Drinking patterns and the composition of the beverage itself both play a role.
But human behavior frequently confounds the precision of science, especially when it comes to alcohol which, in American society, is emotionally laden and (for some people) connected with guilt and sin. Epidemiological studies rely on people to remember things like drinking patterns and report them to researchers.
Scientists say that the emotional baggage of alcohol, combined with memory flaws and selected recall, results in under-reporting of alcohol consumption. People tend to report drinking less frequently than they actually do and they report drinking smaller amounts than they actually consume. This means that a "drink" is probably somewhat larger than the scientific definition. However, if you stick to the scientific definition, then you probably have a safety margin built in.