Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Source : WordNet®

libertarian
     n : someone who believes the doctrine of free will [ant: {necessitarian}]