${\displaystyle \frac{{\text{d}}^{2}x}{\text{d}{t}^2}+2\cdot \vartheta \cdot {\omega }_0\cdot \frac{\text{d}x}{\text{d}t}+{\omega }_0^2\cdot x=0}$

the value of the coefficient $\vartheta$,

where

t	is the time;
x	is a state variable of the system;
ω0	is the characteristic angular frequency of the system

Note 1 to entry: When $\vartheta <1$, ${\omega }_{\text{d}}={\omega }_0\cdot \sqrt{1-{\vartheta }^2}$ is the eigen angular frequency of the system.

Note 2 to entry: This entry was numbered 351-24-18 in IEC 60050-351:2006.