A fluid motion in which the velocities at every point of the field are independent of time; streamlines and trajectories are identical. Sometimes it is further assumed that all other properties of the fluid (pressure, density, etc. ) are also independent of time. All local derivatives in the fundamental equations then vanish. A steady-state solution to a theoretical problem suggests two further questions: how the steady state came to exist (the initial-value problem), and whether it will persist (the instability problem).