Formulated by John Stuart Mill in On Liberty (1859): "The only purpose for which power can be rightfully exercised over any member of a civilized community, against his will, is to prevent harm to others." A person's own good — physical or moral — is not a sufficient warrant for coercion.
The principle is deceptively simple and has supplied the vocabulary for nearly every subsequent liberal argument about paternalism, free speech, drug policy, and sexual morality. Its difficulties are equally famous: what counts as harm? Must it be direct? Does offense count? What about harms to oneself that ripple outward onto dependents or the public purse? Mill himself drew the line narrowly — adults acting on their own account should be left alone even when acting foolishly — but he allowed exceptions for children, "barbarians," and cases where others' interests are genuinely at stake.
Critics from the conservative side (notably James Fitzjames Stephen, and later Patrick Devlin) have argued that societies legitimately enforce shared moral norms even without demonstrable harm. Critics from the left note that the classical liberal picture of the isolated harm-causing individual obscures structural harms — economic, racial, gendered — that don't fit the model.
Still, the harm principle remains the default starting point for liberal political argument in the English-speaking world.