Absolute stability

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).