(I do know what anarcho-capitalism is; I'm anarcho-capitalist myself.)

Libertarianism is a broad term. Anarcho-capitalists are as libertarian as minarchists are. "Libertarian" can refer to either. It's obvious @essrinn was referring to minarchism, but I just wanted to point out that minarchists still advocate presence of a gang with weapons to preserve the "order" in the society and steal from people along the way.