That can be said to be the condition of "rest", after all the changes/alterations were made. This may imply, for examples, that nothing at all happens, or that a "steady" current flows, or that a circuit has "settled down" to final values - that is until the next disturbance occurs.
If the input signal is not time invariant, say if is a sinosoid, the steady state wont be invariant either. The response of a system can be considered to be composed of a transient response: the response to a disturbance, and the steady state response, in the absence of disturbance.
The transient part of the response tends to zero as time since a disturbance tends to infinity, so the steady state can be considered to be the response remaining as T -> infinity.