  Without invoking a perturbative expansion, we define the cosmological
curvature perturbation, and consider its behaviour assuming that the universe
is smooth over a sufficiently large comoving scale. The equations are simple,
resembling closely the first-order equations, and they lead to results which
generalise those already proven in linear perturbation theory and (in part) in
second-order perturbation theory. In particular, the curvature perturbation is
conserved provided that the pressure is a unique function of the energy
density.
