In philosophy, a libertarian is one who holds the incompatibilist philosophy that states that reality is indeterministic, a free action is one which is not determined, and that humans have free will (that is, that there are free human actions). In a way, this means that a free action is simply a random event. The term originated in the 1600s or 1700s.
In general usage, free will usually means is that humans choices are neither random nor determined. This is basically what libertarianism says, but the fact that all free actions are actually random is sometimes put as a stipulation of libertarianism while most people when they say free will will vehemently deny that to be the case. Unfortunately, this is a logical necessity whether we like it or not, so libertarianism is essentially the philosophy held by most people.
cf. determinism, compatibilism
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(This writing has been downvoted a couple times just recently. I'm pretty sure it's all correct, but perhaps I'm mistaken? Maybe the term came about in 12BC? Maybe people object to my using the term 'random'? Maybe people just hate metaphysics, in which case I don't really care. But if it's something I could improve about this writeup, I'd like to know. Thanks.)