a set of variables representing the configuration of the system at time $t$. For instance in a simple mechanical mass-spring-damper system, the two state variables could be the position and velocity of the mass. $mathbf{u}(t)$ is the vector of control inputs at time $t$, representing the externally applied "forces" on the system, and $mathbf{f}$ is a possibly nonlinear function giving the time derivative (rate of change) of the state vector, $dmathbf{x}/dt$ for a particular state, input, and time.