A closed-loop system is absolutely stable if the roots of the characteristic equation have negative real parts.

Equivalently, the poles of the closed-loop transfer
function, or the roots of the transfer function denominator polynomial 1 +
*GH*(*s*), must lie in the left-half plane.

The Nyquist stability criterion establishes the number of
poles of the closed-loop transfer function denominator polynomial 1 +
*GH*(*s*) that are in the right-half plane, directly from the
Nyquist stability plot of the open-loop transfer function
*GH*(*s*).