Just my 2 cents.
My take is that Strangeloop is sort of correct when saying that the world ‘libertarian’ falls under the ‘family resemblance theory’ of Wittgenstein. Can anyone of you give, out of hand, a solid definition of the word ‘furniture’ that covers everything you would call ‘furniture’? Probably not; but it doesn’t mean that you can’t recognize furniture. With this there are some black and white things (an airplane clearly is not furniture, a couch clearly is) but there is some grey (a closet, a cupboard, a bed…?) In any case; with the word ‘libertarian’ there is something similar. Sometimes 2 people with (sort of) the same policies could be considered a libertarian or a free market conservative; depending on his emphasis or depending on the person that is assessing their ideology.
You could call anyone a libertarian who takes the presumption of liberty - i.e. voluntary, contractual relationships and property - and is willing to carefully argue why (in some cases, if any) he differs from it. In this definition; someone like Milton Friedman or Hayek would be libertarians. If you say that a libertarian ‘must’ be someone who (at most) believes in the minimal state (courts, police and national defense); than, obviously, Friedman and Hayek are out. I do think there is no solid definition that everyone agrees too. Most Americans (at fee, Mises, Ihs and students for liberty conferences) I’ve met take one of these 2 (implicit) assumptions. Or this is, at least, my judgement.
Depending on my mood and the person I argue with; I change my take on the word. Because of the way the word is used in my native language (Dutch); I however limit ‘libertarian’ to a smaller tent crowd; while using the word ‘classical liberal’ for the big tent. (Some ‘libertarians’ in this definition are a part of the ‘classical liberal’ movement.)
It’s not theoretically or logically impossible to have a once and for all absolute meaning of the word ‘libertarian’; but in general it is the case that ‘ideologies’ are more like family resemblance definitions that strict definitions.