Transient state

A system is said to be in a transient state when a process variable has been changed and the system has not yet reached a steady state.

Examples

Chemistry

When a chemical reactor is being brought into operation, the concentrations, temperatures, species compositions, and reaction rates are changing with time until operation reaches it nominal process variables.

Electricity

When a switch is flipped in an appropriate electrical circuit containing a capacitor or inductor, the component draws out the resulting change in voltage or current (respectively), causing the system to take a substantial amount of time to reach a new steady state.