For questions about steady states in systems theory, which are unchanging in time.

In systems theory, a system or a process is in a steady state if the variables (called state variables) which define the behavior of the system or the process are unchanging in time.

In continuous time, this means that for those properties of the system, the partial derivative with respect to time is zero. In discrete time, it means that the first difference of each property is zero and remains so: