  The saturation of the magnetorotational (MRI) instability in thin Keplerian
disks through three wave resonant interactions is introduced and discussed.
That mechanism is a natural generalization of the fundamental decay instability
discovered five decades ago for infinite, homogeneous and immovable plasmas.
The decay instability relies on the energy transfer from the MRI to stable slow
Alfv'en-Coriolis (AC) as well as magnetosonic (MS) waves. A second order forced
Duffing amplitude equation for the initially unstable MRI as well as two first
order equations for the other two waves are derived. The solutions of those
equations exhibit bounded bursty nonlinear oscillations for the MRI as well as
unbounded growth for the linearly stable slow AC and MS perturbations, thus
giving rise to the magneto-rotational decay instability (MRDI).
