  A wide range of glassy and disordered materials exhibit complex,
non-exponential, structural relaxation (aging). We propose a simple nonlinear
rate equation d\delta/dt = a [1-exp (b\delta)], where '\delta' is the
normalized deviation of a macroscopic variable from its equilibrium value, to
describe glassy relaxation. Analysis of extensive experimental data shows that
this equation quantitatively captures structural relaxation, where 'a' and 'b'
are both temperature-, and more importantly, history-dependent parameters. This
analysis explicitly demonstrates that structural relaxation cannot be
accurately described by a single non-equilibrium variable. Relaxation rates
extracted from the data imply the existence of cooperative rearrangements on a
super-molecular scale.
