Explain the Nyquist stability criterion.

The Nyquist criterion determines closed-loop stability by examining the open-loop frequency response plot in the complex plane. The Nyquist plot is G(j*omega)*H(j*omega) for omega from -infinity to +infinity. The system is stable if the number of counterclockwise encirclements of the critical point (-1,0) equals the number of right-half-plane open-loop poles. For most systems, zero encirclements means stability. Nyquist handles open-loop unstable systems and systems with time delay, which root locus cannot address directly.