by Bryan Caplan
This is actually a complicated question, because the term "libertarianism" itself has two very different meanings. In Europe in the nineteenth century, libertarianism was a popular euphemism for left-anarchism. However, the term did not really catch on in the United States.
After World War II, many American-based pro-free-market intellectuals opposed to traditional conservatism were seeking for a label to describe their position, and eventually picked "libertarianism." ("Classical liberalism" and "market liberalism" are alternative labels for the same essential position.) The result was that in two different political cultures which rarely communicated with one another, the term "libertarian" was used in two very different ways. At the current time, the American use has basically taken over completely in academic political theory (probably owing to Nozick's influence), but the European use is still popular among many left-anarchist activists in both Europe and the U.S.
The semantic confusion was complicated further when some of the early post-war American libertarians determined that the logical implication of their view was, in fact, a variant of anarchism. They adopted the term "anarcho-capitalism" to differentiate themselves from more moderate libertarianism, but were still generally happy to identify themselves with the broader free-market libertarian movement.