The conception of freedom as self-mastery, autonomy, or collective self-determination — the capacity to act as one's "true" or rational self would, rather than merely the absence of external constraint. The distinction from Negative Liberty was made canonical by Isaiah Berlin in Two Concepts of Liberty (1958), though both traditions are much older.
The positive conception has a deep and largely continental pedigree. Jean-Jacques Rousseau's "forcing to be free" through the general will, Kant's autonomy as obedience to self-legislated reason, G. W. F. Hegel's concrete freedom realized in rational institutions, and Marx's emancipation from capitalism's structural unfreedom all belong to it. Its paradigm is not the silent space around the individual but the achievement of a condition — self-government, rational self-direction, emancipation — in which one is meaningfully in charge of one's own life.
Berlin's worry, writing in 1958 with totalitarianism fresh in view, was that positive liberty invites a split between the empirical self (what I actually want) and the "true" self (what I rationally should want) — and that once you admit this split, regimes can claim to coerce people in the name of their deeper freedom. The 20th century provided plenty of examples.
Defenders — especially republican theorists like Quentin Skinner and Philip Pettit — reply that this conflates positive liberty with its worst interpretations. A richer account of freedom as non-domination, autonomy, or real effective capacity (think of Amartya Sen's "capabilities") need not slide into authoritarianism. The debate between the two conceptions structures most contemporary liberal-vs.-republican argument about what freedom is actually for.