A measure of the relative stability of a system, defined as the reciprocal of the gain of the system at the frequency at which the phase angle reaches 180°. The gain margin is a measure of the factor by which the system gain would have to be increased for the locus on a polar plot (phase against magnitude) to pass through the −1 point. This can be related to the (−1, 0) point – the Nyquist point – on the polar plot (see diagram). In the diagram, the locus a has a gain margin of 1/0.5 = 2, the locus b has a gain margin of 0.

Polar plot showing gain margin