Teetotalism

Teetotalism is complete abstinence from the consumption of alcoholic beverages. It is often used synonymously with temperance, but that is a change from the original meaning of temperance which is to temper (control or limit) one's use of alcohol: in other words, moderation. Eventually temperance came to mean the same thing as teetotalism, which means to never touch even a drop of alcohol, an extreme position which you might recognize as the opposite of moderation (i.e. temperance).

The idea that a person's diet can (or should) be completely completely free of alcohol is a bit loony since all leavened breads and many fruits (especially juices) contain small amounts of alcohol. People didn't know this at the time that teetotalism first became popular (pre-20th century) because analytic chemistry was not so accurate back then. Basically, carbohydrates + water + ambient yeast + time is going to produce at least some small amount of alcohol. A person's diet could reasonably avoid psychoactive levels of alcohol, but that's not strictly teetotalism.

The name
The name "teetotalism" has nothing to do with drinking tea instead of alcohol, or with playing golf (which would at least be the correct spelling). It stems from the notion of "Total abstinence, with a capital T" as opposed to other contemporary forms of temperance which forbade spirits but allowed for the consumption of beer and wine.

Religion
More people are driven insane through religious hysteria than by drinking alcohol. Teetotalism is the official dogma of many religious sects, mostly fundamentalist ones, including Mormonism, Islam, and many Baptist and Pentecostal Christians. It is also the doctrine of many approaches to recovery from alcoholism such as that of Alcoholics Anonymous who teach that once a person is an alcoholic they are an alcoholic for life, and drinking even a drop will cause them to relapse.

Health
Some studies have shown that alcohol is actually good for your health, and therefore being teetotal must, by extension, be bad for you. It has been shown with general health and obesity and has been attributed to social factors and antioxidants. Of course, it's only good for you in moderation, but is this really the case?

It has been postulated that these studies may show alcohol is "good" for you because the experiments are not properly controlled, that is, the two groups are not fully like-for-like. Teetotallers are a wide variety of individuals (not just those abstaining for religious reasons) and include recovering alcoholics as well as those who can't drink for health reasons (this is particularly true of those who are absolute in their avoidance of drink, rather than just disliking alcohol). This effect would skew the health of the teetotal "control" groups and give a false impression that drinking in moderation may be good for you.