A circuit whose parameters or conditions remain constant is said to be in a steady state. A circuit isn't always in a steady state. When a circuit or a portion of it is switched on or off there is a sudden change. The amount of time taken for these changes to happen is called the Transient period, with this phenomenon called Transient.

After the transiency period, the circuit settles down and attains steady state if not disturbed further.

When you switch on a circuit there are two responses, one is the transient(natural) response and the other is the steady state/forced response.

Transients are due to the presence of energy storing elements like capacitors and inductors which initially oppose changes in values and then settle down.