My own definition is that in order to be considered a libertarian, at least in the political context, an individual must adhere without compromise to five key points. Ideally, of course, we'd all be in agreement on everything. But we're not, and probably never will be. Debate is likely to continue indefinitely on such matters as abortion, foreign policy, and whether, when, and how various government programs can be discontinued or privatized.
Slavery is wrong, period.
Because you own yourself, you are responsible for your own well-being. Others are not obligated to feed you, clothe you, or provide you with health care. Libertarians yield to no one in their support for our right as individuals to keep and bear arms. But criminal possession laws are an affront to liberty, whatever the rhetoric used to defend them. In an ideal world, there would be no taxation.