A political philosophy that seeks maximal individual autonomy with a strong emphasis on political freedom, freedom of choice, individualism, and voluntary association.
“Libertarianism holds that the only proper role of violence is to defend person and property against violence, that any use of violence that goes beyond such just defense is itself aggressive, unjust, and criminal” -Murray Rothbard
Contemporary libertarianism is most often associated with the non-aggression principle, an Austrian stance on economic policies, and anarchical philosophies such as anarcho-capitalism, though the depth of the association tends to vary depending on the use-case.
Libertarian doctrine traces its roots back to early enlightenment philosophers such as Leveller and John Locke in their publications against the established monarchies of their time. The continual development of early libertarianism is directly intertwined with the emergence of early anarchism, with a strong influence from Rousseau's arguments for "the centrality of freedom".
Contemporary American Libertarianism first emerged in the early 20th century with writers such as H.L. Menken and Robert D. Nock who engaged in criticisms of the continual federal centralization of America across a number of publications. Ayn Rand however is ultimately credited with the revival of libertarianism in American through the introduction of her own individualist philosophy and the inspiration it provided for future libertarians (though it is important to note that Rand was staunchly opposed to libertarianism and anarchical philosophy).
Post-WWII, Murray Rothbard, Karl Hess, Samuel Edward Konkin III, and Robert Nozick laid the foundation for the intellectual development of libertarianism to its modern position including now staple ideas such as the non-aggression principle. In 1971 David Nolan founded the Libertarian Party in the United States which attempted to adapt the ideals of libertarianism to the national political scale, though not without its staunch critics.
As advocates of both aspects of libertarianism utilize the moniker of "libertarian" the two ideas are often conflated as being analogous, where either the politics directly reflect the philosophy, or the philosophy directly reflect the politics, both of which are rarely if ever true.
The divide of "libertarians" can primarily be attributed to this unintuitive distinction. Colloquially in modern times, this has been denoted by introducing yourself as a "small l" or "Big L" libertarian to identify your attachment to the philosophy or political party respectively. Without a clear way to separate the two notions and with the political sphere having priority in the public consciousness this has lead to many who would fit into the category of a philosophical libertarian abandoning the term to move to other closely related or analogous philosophies such as anarchism, anarcho-capitalism, agorism, etc.
Several "principle oriented" caucuses have been developed to attempt to address the issue of the separate notions between the two groups, most notably the Libertarian Party Mises Caucus - however, for some, such as libertarian anarchists, the involvement in politics altogether (with some specifically refuting the involvement in national politics) is not in alignment with fundamental libertarian philosophy regardless of any particular groups proximity to other fundamental tenants of libertarianism.