The Collaborative International Dictionary of English v.0.48:

Libertarian \Lib`er*ta"ri*an\, n.
   One who holds to the doctrine of free will.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Libertarian \Lib`er*ta"ri*an\ (-t[=a]"r[i^]*an), a. [See
   Liberty.]
   Pertaining to liberty, or to the doctrine of free will, as
   opposed to the doctrine of necessity.
   [1913 Webster]

WordNet (r) 3.0 (2006):

libertarian
    n 1: an advocate of libertarianism
    2: someone who believes the doctrine of free will [ant:
       necessitarian]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "libertarian":
   big person, broad-gauge person, broad-minded, ecumenicist,
   free lance, free spirit, free trader, freethinker, freethinking,
   independent, individualist, isolationist, latitudinarian, liberal,
   liberalist, liberalistic, libertine, mugwump, neutral, nonpartisan,
   open-minded, rugged individualist, third force, third world,
   tolerant, unbigoted