The conception of freedom as the absence of external interference: you are free to the extent that no other person prevents you from acting as you might choose. The phrase was made canonical by Isaiah Berlin in his 1958 lecture Two Concepts of Liberty, which distinguished it from "positive liberty" — freedom understood as self-mastery, or as acting in accordance with one's "true," rational, or collective self.
The distinction is more than semantic. Negative liberty is the tradition of John Locke, John Stuart Mill, and Classical Liberalism: its paradigms are the The Harm Principle, individual rights, and the silent space around a person into which the state may not step. Positive liberty, by contrast, runs through Jean-Jacques Rousseau, Hegel, and Marxist and republican traditions: its paradigms are self-government, autonomy, and emancipation from internal or structural constraints. Berlin's worry — writing in the shadow of mid-century totalitarianism — was that the positive conception, by distinguishing the "true" self from the empirical one, licensed regimes to coerce people "for their own good" in the name of their deeper freedom.
Later critics (Charles Taylor, Quentin Skinner, Philip Pettit) have pushed back from multiple directions. Republican theorists in particular have argued for a third conception — freedom as non-domination — that cuts across Berlin's binary. But for most Anglophone liberal political argument, the negative-liberty default is still the starting point.