 Quality factor and viscous damping regimes

For a linear oscillator with viscous drag, this parameter puts the underdamped/critical boundary at $Q=1/2$. The energy-per-cycle interpretation and narrow displacement linewidth are most useful at high quality factor; critical and overdamped free motion do not complete oscillation cycles. Amplitude and averaged-energy ringdown times differ by a factor of two. The damping classification is not the same as the condition for a nonzero-frequency displacement maximum.