A or more generally a transfer element with an input variable and an output variable is referred to as unstable (not stable from a transmission perspective) if the output variable associated with each restricted input variable is not restricted as well at all times t ≧ t 0 (initial position of rest) (as per DIN 19 229).
A distinction is made between "monotonous instability", when the system moves away from the initial position in a uniform manner following an impulse (Figure I 14 a), and "oscillatory instability" whereby the system experiences oscillations of ever-increasing amplitude (b).
Figure I 14: Instability associated with oscillation