Transient means passing with time. For example, a damped oscillator needs some time after a temporary disturbance to reach the equilibrium again, and after a permanent change in system variables, to reach the new equilibrium.
In Electrical engineering
A transient system is a short-lived oscillation in a system caused by a sudden change of voltage, current or load. Engineers use voltage regulators and surge suppressors to prevent transients in electricity from affecting delicate equipment.
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Source: CDX Global & Wikipedia - en.wikipedia.org
