After much thought and debate about this topic over the last 25 or so years, here is my attempt at a lean, concise, precise definition of what a libertarian is:
A libertarian is a person who believes that the invasion of the borders of (trespass against) others’ bodies or owned external scarce resources, i.e. property (with property allocations determined in accordance with Lockean homesteading rules and contractual transfer rules), is unjustified, because they (for whatever reason) prefer or value grundnorms of peace, prosperity, and cooperation and who have enough honesty, consistency, and economic literacy to recognize that the libertarian assignment of property rules is necessary to achieve these grundnorms.
Such a person, if he is consistent, also cannot help but recognize that the state, being an agency of institutionalized aggression, is inherently criminal and illegitimate.
Note what this does not say: It does not say that the libertarian necessarily believes all aggression is immoral, but rather that it is unjustified; it does not imply that rights are a “subset” of morals. It also does not say why the person values peace, prosperity and cooperation and favors it above interpersonal violent conflict. It also does not make the common mistake of interpreting the libertarian-Lockean property allocation rule as requiring one to prove title all the way back to the very first use of the resource; rather, it says that whoever has the best claim to a disputed resource has a property right in it (is its “proper” owner), and that as between any two claimants, the one having an earlier claim (use) of the property has the better claim. This does not require title to be traced back to the beginning of time but only to the earliest time needed to defeat any actual or potential claimants; though it implies that someone who can trace title back to the first appropriation has the best possible claim of all (unless title has been assigned by contract). Note also that although the libertarian rule is the Lockean rule this does not imply Locke’s reasoning in justifying his homesteading rule was correct—in particular it does not imply that Locke was right to say that labor is owned or that labor-ownership is the reason why first possession of a resource is sufficient to establish property rights in the resource.
I like it. He follows it with some resource links. Click the link above for those.