1. (n.) One who believes that the ultimate good in a political system is to keep government regulation at an absolute minimum, particularly by eliminating laws based on their validity (or lack thereof) within a free society.
2. (n.) A member of the U.S. political party of the same name.
3. (adj.) the state of adhering to the principle of libertarian politics. "This bill is based on libertarian values."
Libertarians contrast with the Republican and Democratic parties because the libertarian belief is that government should be minimized or eliminated in all areas, even those that they don't personally support, while other political parties tend to promote freedom in some issues but government control in others. While the Democratic party believes in freedom for matters of recreational drug use and the Republican party believes in the freedom to own guns, libertarians are typically against regulation for both issues.
Libertarians themselves do not all necessarily share equal standards for personal behavior (though some may be common), as their philosophy is based almost entirely on knowing when something isn't anybody else's business. For example, many do not think recreational drug use is a good idea, but do not want to enact laws making that choice for others.