Nyquist Stability Criterion

A frequency-domain stability test for closed-loop feedback systems which maps the open-loop transfer function around a contour encircling the right half-plane and counts net encirclements of the critical point −1 to determine the number of closed-loop poles in the right half-plane, taking into account poles of the open-loop in the right half-plane via the argument principle.

A frequency‑domain test for linear time‑invariant (LTI) control systems that determines the number of closed‑loop poles in the right‑half complex plane by mapping the open‑loop transfer function evaluated on a Nyquist contour into the complex plane and counting encirclements of the critical point −1.