Lib`er*ta"ri*an (-tā"r&ibreve;*an), a. [See Liberty.] Pertaining to liberty, or to the doctrine of free will, as opposed to the doctrine of necessity.

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

• One who advocates liberty either generally or on a specific issue, e.g. "civil libertarian" (in favour of civil liberties).
  
• (Chiefly US) A believer in a political doctrine that emphasises individual liberty and a lack of governmental regulation and oversight both in matters of the economy ('free market') and in personal behavior where no one's rights are being violated or threatened. Also 'classical liberal', akin to 'anarcho-capitalist'.
  
• In the philosophy branch of metaphysics, a believer in thinking beings' freedom to choose their own destiny, i.e. a believer in Free Will as opposed to those who believe the future is predetermined.
  
• (US)(prefixed to 'Republican'): a member of the Republican Party (especially a legislator) who emphasizes economic and Constitutional, rather than religious and personal, aspects of the party's platform.
  
• Capitalised: A member of a political party or movement that uses the term "Libertarian" in its name (e.g., the Libertarian Party of the United States), and is likely to support policies that are libertarian in sense number 2).
  
• Relating to the beliefs of libertarians, as in, "He has libertarian views". A relative tendency towards liberty, as in "libertarian socialist".